单项选择题
A．voluntary muscles B．involuntary muscles C．muscle fibers D．sensory nerves
A．the efficiency of our muscles B．the normal breathing function C．directing our muscles intelligently D．the work of only our involuntary muscles
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