单项选择题

Scientists ______that bacteria through microscopes for many years．

A．studied
B．have studied
C．had studied
D．have seen studied
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单项选择题
______ we need air and water, we need criticism and self criticism.
A. Even though
B. So long as
C. Just as
D. Now that
单项选择题
I suggested that the students each ______a plan for the summer vacation．
A．would make
B．will make
C．make
D．made
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