Wind power is an ancient source of energy we may return in the near future.
A. to which
B. by which
C. from which
D. on which
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(答案→)A. to which
B. by which
C. from which
D. on which
解析： 这题考察定语从句的用法，介词+关系代词的用法，代词的选用可以根据先行词，也可以根据定语从句的动词搭配，这题定语从句中的turn 和to搭配，就是说，which前面加to，句意是：风能是一种古老的能源，我们在不久的将来可能会回头利用它。选A。