问题补充：
It suddenly occurred to him he had left his keys in the office.
A. whether
B. where
C. which
D. that
答案：
A. whether
B. where
C. which
(答案→)D. that
解析：句意为: 他突然想起钥匙忘在办公室里。it 做形式主语，he had left his keys in the office 是后置的主语从句。成分完整，不含疑问语气。所以用that 连接。
【考点定位】sth occurs/occurred to sb某人突然想起……事(物)做主语。如 An idea occurrs to me that the lights in the classroom have been on.
