单项选择题

______you understand this rule, you'll have no further difficulty.

A．Once
B．Unless
C．Only
D．Until
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单项选择题
There are no lights on and no one answer the doorbell. They ______ be at home.
A．must not
B．will not
C．should not
D．can not
单项选择题
The book ______ by a famous writer.
A．was believed to write
B．believed to be written
C．was believed to be written
D．believe to write
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