单项选择题

It ______ seen to be believed.

A．has
B．has been
C．has to be
D．was
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单项选择题
Dr. Smith failed many times but he finally ______ to find a successful way to solve the problem.
A．broke off
B．broke down
C．broke out
D．broke through
单项选择题
80（）

A．But
B．For
C．Still
D．And
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