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