When last ____valuable,those books enjoyed great success.

题目

When last ____valuable,those books enjoyed great success.
A.finding B.found C.being found D.to be found

答案

这道题目,我们要注意分析句子结构,主语是those books,所以这里逗号前面的应该用被动语态,A首先排除,而C是表示正在被发现,也不对,排除.D,这里只是表示一种状态,没必要用动词不定式,也排除,所以选B.
把句子换一种方式表达就是：When those books were last found valuable,those books enjoyed great success.这样你肯定能够看明白.