              
                                  

                       
                                                                                                                            
                                                                                                                                                                                                           
    20                                                     1980                             1993   4                                             
      
       
     
      
       
       
     
       
     
      
     
      
     
     
     
      
     
       
      
       

          
                  
                                                                                                                                   
                 
                                                                                          
                  
                                                                                                                        
                 
                                                                                   
                                                                   
          
                
                 
                
                 
               
                                                                      
                                                                                                                                     
                
                                                                                                       
                                                                                                                                   
                 
                                                                                                                                                          
                  
                                                                    