问题补充：
He said that the key to open the door had been lost.
A. with what
B. with it
C. with that
D. with which
答案：
A. with what
B. with it
C. with that
(答案→)D. with which
解析：考查prep+which+to do sth.作定语的用法。再如：He had to find a house in which to store his books.
