31.A (A strong mind needs a strong body in order to make it most useful．）
32.C （The air in the house doesn't remain fresh very long．But out of doors it is usually fresh and pure）
33.C （Do not neglect your body．It is as much important as your mind．）
34.C（system的意思是系统。根据原文“It is because in fresh air more oxygen is taken into our systems and more wastes are given off．”译为“这是因为在新鲜的空气中我们的身体可以吸入更多氧气并且排除更多垃圾”）
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