53 .(    )

10. Only then (   ) that he had left his key on his office. 

• A、he realized
  
  

• B、did he realize
  
  

• C、had he realized
  
  

• D、he had realized
  
  

40. What does the passage talk about?

• A、Warfare is a"law of nature". 
  
  

• B、Warfare is not a"law of nature". 
  
  

• C、Warfare controls human population. 
  
  

• D、None of the lowlier animals, except ants, practice warfare upon their own species.