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：文中指出to the exact extent necessary for each action you perform，可知C项符合。to the extent某种程度。

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

【正确答案】 C

【答案解析】解析：由“More than half of a person’s body is composed of muscle fibers”可知C为正确答案。

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：由“Women have only 60 to 70 percent as much muscle as men for their body mass．”可知B为正确答案。

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：由“The sixth is a muscular sense responsible for directing your muscles intelligently”可知C为正确答案。

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：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C不是肌肉的特性。