资料题：We may assume from the passage that young children（　）。

Physical fitness is the result of many factors-good medical and dental care, proper nutrition, adequate rest and relaxation, and sensible personal habits.But these are never enough.An essential factor is regular physical activity-exercise for a body that needs it to function well.All of us know that ordinary movements-things like running, jumping and bending- are made by muscles.Muscles also do many other things.They suck air into your lung, push food along your digestive tract, and tighten your blood vessels to raise blood pressure when you need more pressure for an emergency.Your heart is a muscular pump.When your muscles are not used, or are not used enough, they do not develop and grow properly.They soon become flabby and weak, shrink in size, and lose the ability to do the jobs they were meant to do.Recent studies showed that the average high school student spends fifteen to thirty hours aweek watching television and only two hours a week in organized play or exercise.That is one reason why one - third of the 200,000 pupils tested far the President's Council on Physical Fitness failed a simple test of strength, stamina, and flexibility.Some of the boys and girls could not raise themselves to a sitting position without using their hands!Cars and buses, elevators and escalators, TV and push - button machines all tend to reduce strenuous physical activity.But our bodies still need exercise.Without it they soon become weak, ineffective, and unattractive.