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

发布时间：2020-08-17 01:22:06

Qian Xuesen tried many times      he found the solution to the puzzle about space technology.
A. after
B. before
C. since
D. when

网友回答

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。
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