46. What would be the most suitable title for the text?

• A、Zoos---Best Places for Wild Animals
  
  

• B、Is It Right to Keep Animals in Zoos?
  
  

• C、Zoos--- Places for Fun to the Public
  
  

• D、How Should Animals Be Treated in Zoos?
  
  

21.

• A、because
  
  

• B、where
  
  

• C、once
  
  

• D、although
  
  

 52.