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

单项选择题Muscles are unique fibers, because they can（）

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

单项选择题Intelligent use of the muscles means that （）

A．one always knows what his muscles are doing
B．one performs simple actions without working
C．one's muscles are used only to the extent necessary for each action they perform
D．one improves muscular action consciously