单项选择题

Training astronauts（）not an easy thing.

A．are
B．is
C．were
D．be

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单项选择题

Look at the terrible situation I am in ! If only I（）your advice.

A．had followed
B．would follow
C．follow
D．have followed

单项选择题

I invited Tom and Ann to dinner, but（）of them came.

A．both
B．none
C．either
D．neither

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