问题补充：
Wind power is an ancient source of energy we may return in the near future.
A. on which
B. by which
C. to which
D. from which
答案：
A. on which
B. by which
(答案→)C. to which
D. from which
解析：句意：风力是古老的能源，我们不久的将来也许会再次回到风力上来。Return to回到，返回；这里which引导的是定语从句，它代替source of energy，故选C。
