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
It is necessary for technicians ______ one or two foreign languages．
A．master
B．to master
C．mastering
D．mastered
单项选择题
Sickness deprived me ______ the pleasure of seeing you.
A．from
B．of
C．out of
D．with
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