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

• A、the second sentence
  
  

• B、the fifth sentence
  
  

• C、the seventh sentence
  
  

• D、the forth sentence
  
  

55. (    )

15. (   ) he has limited technical knowledge, the old worker has a lot of experience. 

• A、Although
  
  

• B、Unless
  
  

• C、As
  
  

• D、Since