Qian Xuesen tried many times he found the solution to the puzzle about space technology.A

问题补充：

Qian Xuesen tried many times he found the solution to the puzzle about space technology.

A. after

B. before

C. since

D. when

答案：

A. after

(答案→)B. before

C. since

D. when

解析：after在……之后；before在……之前；since自……以来；when当……时候。

句意：钱学森在他发现航天技术之前，他尝试了许多次。