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 _________.

　　A.be overweight

　　B.lacking adequate exercise

　　C.hurt badly

　　D.not used