单项选择题

Suddenly he heard someone ______ gently on the window.

A．knock
B．knocking
C．knocked
D．to knock
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单项选择题
Mr. Johnson preferred ______ heavier work to do.
A．to be given
B．to be giving
C．to have given
D．having given
单项选择题
It is extremely urgent that they ______ from the mountain before dark.
A．must be rescued
B．should rescue
C．can be rescued
D．be rescued
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