单项选择题

Not until three years ago ______ to work outside.

A. he began
B. he begins
C. began he
D. did he begin
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单项选择题
The student is shy and he ______ questions in class.
A. dares not ask
B. does not dare to ask
C. dares not to ask
D. does not dare ask
单项选择题
Most environmental problems exist because adequate measures for preventing them ______ taken in the past.
A. was not
B. were not
C. were not being
D. being not
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