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.

【正确答案】 C

【答案解析】解析：题目问的是肌肉的合理使用是什么意思，文中指出“to the exact extent necessary for each action you perform”可知，是精确地执行每一个动作，故选C。

A、 Voluntary muscles.
B、 Involuntary muscles.
C、 Muscles fibers.
D、 Sensory nerves.

【正确答案】 C

【答案解析】解析：题目问的是人体一半以上是由什么组成的，由“More than half of a person’s body is composed of muscle fibers”可知一半以上是肌肉纤维，故选C。

A、 Because she has more voluntary muscles than an average man.
B、 Because she has only 60 to 70 percent as much muscle for her body mass as man's.
C、 Because she has 60 to 70 percent less muscles for her body mass.
D、 Because she has less muscle fiber for her body weight.

【正确答案】 B

【答案解析】解析：题目问的是为什么女人没有男人强壮，由“Women have only 60 to 70 percent as much muscle as men for their body mass”可知，女人身体的肌肉纤维只有男人的百分之六七十，故选B。

A、 The efficiency of our muscles.
B、 The normal breathing function.
C、 Direction our muscles intelligently.
D、 The work of only our involuntary muscles.

【正确答案】 C

【答案解析】解析：题目问的是肌肉觉负责的是什么，由“The sixth is a muscular sense responsible for directing your muscles intelligently”可知，肌肉觉负责指导肌肉的合理使用，故选C。

A、 They can become shorter and thicker.
B、 They can stretch.
C、 They have sense linking the muscles to the brain.
D、 They can retract to their original position.

【正确答案】 C

【答案解析】解析：题目问的是哪一项不是肌肉的特点，从“They have three unique characteristics：they can become shorter and thicker；they can stretch，and they can retract to their original position”可知，没有提到C选项，故选C。