题目内容：

It is not always easy for the public to see _______ use a new invention can be of to human life.
A.what B.that C.how D.which

优质解答

选D
首先判断出can是谓语,前面 必须是主语,排除that（不能做主语）和how（后面要加to）
这里面如果用which是因为有”use a new invention“这样的动词修饰,而what不能被这样修饰