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
网友回答
A. on which
B. by which
(答案→)C. to which
D. from which
解析：考查定语从句。本题中的介词to是从动词return后面移至关系代词which前面。句意：风能是一种古老的能量来源，在不久的未来我们就会重新使用它。Which指代前面的先行词an ancient source of energy作为介词to的宾语。动词return与介词to连用，故C正确。