Only when he arrived home that he had left his key in the office.
A. will he realize
B. he did realize
C. did he realize
D. should he realize
网友回答
A. will he realize
B. he did realize
(答案→)C. did he realize
D. should he realize
解析：only+状语主句采用部分倒装，本题叙述的是过去的事情所以应该用一般过去时态答案为D。