Scientists overcame (克服) much difficulty they had _____ that

Scientists overcame (克服) much difficulty they had _____ that technological problem _____ they finally
could send the manned (载人的) spaceship up into space.
[ ]
A. to solve; until
B. solving; before
C. to solve; before
D. solving; until
biliping 1年前 已收到1个回答 举报

201ren 幼苗

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B

1年前

8
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