单选题
The student asked her professor if he would have gone on the space ship {{U}}he did know earlier{{/U}}.
A、
if he knew
B、
if he knows
C、
he had known
D、
had he known
【正确答案】
D
【答案解析】
[分析] 虚拟语气。 本句是对过去事实的假设，条件从句的谓语应用过去完成时(had known)，而if条件句中如有were，should，had，可将if省去，而把were，should，had放在句首。因此选项D是答案。
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