单项选择题

According to the passage, movement occurs when a muscle______.

A．grows
B．shortens
C．relaxes
D．pushed
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单项选择题
Why are some muscles attached to bones at an angle
A．To cause the muscles to push.
B．To strengthen the muscles.
C．To shorten tissues.
D．To perform a turning action.
单项选择题
The passage implies that reversing the muscle-bone movement used to bend an arm requires______.
A．the relaxation of two sets of muscles
B．the use of different muscles
C．a different kind of tissue
D．the enlargement of the muscle
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