Scientists overcame (克服) much difficulty they had _____ that technological problem _____ they finally
could send the manned (載人的) spaceship up into space.

A. to solve; until
B. solving; before
C. to solve; before
D. solving; until
B
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Scientists overcame (克服) much difficulty they had _____ that technological problem _____ they finally
could send the manned (載人的) spaceship up into space.
[     ]
A. to solve; until
B. solving; before
C. to solve; before
D. solving; until

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