单项选择题

Under a microscope , muscle cells appear to be （）

A．textured like wood
B．colored like wood
C．smooth and red
D．short and thick

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单项选择题
According to the selection more than half of a person's body is composed of （）

A．voluntary muscles
B．involuntary muscles
C．muscle fibers
D．sensory nerves
单项选择题
Muscles are unique fibers, because they can（）

A．contract
B．stretch
C．retract
D．do all of the above
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