51.(      )

17. The park is to a city (   ) the lung is to man' s body. 

• A、as
  
  

• B、that
  
  

• C、which
  
  

• D、what
  
  

二、詞匯與語法知識: 共15小題; 每題2分, 共30分。從每小題的四個選擇項中, 選出最佳的一項。

6. Of all (   ) reasons for my decision to become a university professor, my father's advice was most (   ) important one. 

• A、the; a

• B、不填; a

• C、不填; the

• D、the; the