A、Voluntary muscles. B、Involuntary muscles. C、Muscles fibers. D、Sensory nerves. C题目问的是人体一半以上是由什么组成的。由定位句可知，一半以上是肌肉纤维，故选C。

Are you aware that you actually possess six senses? The sixth is a muscular sense responsible for directing your muscles intelligently—to the exact extent necessary for each action you perform. For example, when you reach for an object, the sensory nerves linking the muscles to the brain stop your hand at the correct spot. This automatic perception of the position of your muscles in relation to the object is your muscular sense in action.
    Muscles are stringy bundles of fibers varying from one five-thousandth of an inch to about three inches. They have three unique characteristics: they can become shorter and thicker; they can stretch; and they can retract to their original position.
More than half of a person’ s body is composed of muscle fibers, most of which are involuntary—in other words, work without conscious direction. Women have only 60 to 70 percent as much muscle as men for their body mass. That is why an average woman can’ t lift as much, throw as far, or hit as hard as an average man.
    Q28: What does intelligent use of the muscles mean?
    Q29: What is composed of more than half of a person’ s body?
    Q30: Why is an average woman weaker than an average man?