15. Jack was happy that the train (   ) when he (   ) the station. 

• A、left; had arrived
  
  

• B、had not left; arrived
  
  

• C、had not left; had arrived
  
  

• D、left; arrived
  
  

52 .(    ).

42. What does the "law of nature"mean? You can find the answer in the passage in (   ).

• A、the first sentence
  
  

• B、the fourth sentence
  
  

• C、the seventh sentence
  
  

• D、the final sentence