单项选择题

Scientists say it may be a few years（）it is possible to test the new medicine on patient.

A. because
B. after
C. before
D. since

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单项选择题
The boy spends at least as much time reading as he （）

A. writes
B. is writing
C. does to write
D. does writing
单项选择题
— What was he doing at this time yesterday — He （） to sing a song in English.

A. was trying
B. is tried
C. is trying
D. was tried
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