单选题 The student asked her professor if he would have gone on the space ship he did know earlier.
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】[解析] 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。