单项选择题

______ the reactions of people with this drug, scientists have learned more about the brain.

A．By studying
B．To study
C．They study
D．They're studying
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单项选择题
They insisted that we ______ a delegation to their country.
A．sent
B．would send
C．send
D．to send
单项选择题
Tom ______ have met us three times last year, but' the first two meetings were canceled.
A．will
B．should
C．had
D．must
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