单项选择题

______, he can quickly find out what's wrong with the machine.

A．Having been well trained
B．Having well trained
C．To have been well trained
D．To have been well trained
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Which of the following helps to break a fall from a second floor window
A．Cement.
B．Grass.
C．Pavement.
D．Concrete.
单项选择题
What sense is weak or nonexistent among birds
A．Hearing.
B．Sight.
C．Smell.
D．Taste.
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