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.