_________ we must do is _________ the use of energy．

_________ we must do is _________ the use of energy．

A.That; to reduce

B.How; to reduce

C.What; reduce

D.That; reduce

答案及解析： C

解析：

what引导主语从句，作do的宾语；

因主语从句中谓语动词为do，则不定式作表语时可省略to。