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 stringly 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 positions. Under a high-powered microscope， muscle tissue is seen as long， slender cells with a grainy texture like wood. 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. The voluntary muscles， those that we move consciously to perform. particular actions， number more than five hundred. 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. According to the selection， the muscular sense is responsible for ______. A.the efficiency of our muscles B.the normal breathing function C.directing our muscles intelligently D.the work of only our involuntary muscles Intelligent use of the muscles means that ________.A.one always knows what his muscles are doing B.one performs simple actions whithout working C.one‘s muscles are used only to the extent necessary for each action they perform D.one improves muscular action consciously Muscles are unique fibers because， they can ________.A.contract B.stretch C.retract D.do all of the above Under a microscope， muscle cells appear to be _______.A.textured like wood B.colored like wood C.smooth and red D.short and thick 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