It suddenly occurred to him he had left his keys in the office.
A. whether
B. where
C. which
D. that
网友回答
A. whether
B. where
C. which
(答案→)D. that
解析：考查固定结构。固定结构it occur to sb that…某人突然想去某事；it是形式主语，真正的主语是that引导的主语从句。That没有任何意思只是在句中起引导作用。句意：他突然想起他把钥匙忘记在办公室里了。故D正确。