10. My father knows Kate's father. They (   ) to each other at a conference. 

• A、had been introduced
  
  

• B、are introduced
  
  

• C、were introduced
  
  

• D、have been introduced
  
  

52. (     ) 

30. (     )

• A、without
  
  

• B、till
  
  

• C、for
  
  

• D、on