These diagrams are especially helpful when we have a concrete problem at hand.
A. be solved
B. to solve
C. solved
D. being solved
网友回答
A. be solved
(答案→)B. to solve
C. solved
D. being solved
解析：句意为：当我们手边遇到具体的问题时，这些图表特别有用。本结构中如果满足两个条件时则用主动表被动的形式，一个条件是a concrete problem与solve构成了被动关系，另一个条件是we与solve 之间构成了主动关系，这时候要用主动表被动所以答案应为C。而如果是下面这种情况：-----Master, do you have any clothes (wash)? -----Yes. 在这一题目中，是仆人为主人洗衣服，虽然any clothes与wash之间构成了被动关系，但不是you去洗，故应为被动形式，所以应为to be washed.