[单选题]【完成句子】 Recent studies showed that the average high school student spends fifteen to thirty hours a week 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 for 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! Some people have difficulties in standing up due to _________.