第6部分：完形填空
阅读下面的短文，文中有15处空白，每处空白给出了4个选项，请根据短文的内容从4个选项中选择1个最佳答案。
Exercise
Whether or not exercise adds __1__ the length of life，it is common experience that a certain__2__of regular exercise improves the health and contributes a feeling of well-being．Furthermore, exercise__3__involves play and recreation，and relieves nervous tension and mental fatigue in so doing, is not only pleasant but beneficial.
How much and what kind of exercise one should __4__ merits careful consideration．The growing child and the normal young man and young woman thrill with the exhilaration of strenuous sports． They fatigue to the__5__of exhaustion but recover promptly with a period of rest．But not so with__6__ of middle age and beyond．For them moderation is__7__ vital importance． Just how much exercise a person of a given age can safely take is a question __8__to answer．
Individual variability is__ 9__ great to permit of generalization．A game of tennis may be perfectly safe for one person of forty but folly for another．The safe limit for exercise __10__ on the condition of the heart，the condition of the muscles，the type of exercise，and the regularity with which it is taken．Two general suggestions，however，will __11__ as sound advice for anyone．The first is that the condition of the heart and general health should be__12__ periodically by careful，．thorough physical examinations．The__13__ is that exercise should be
kept below the point of physical exhaustion．
What type of exercise one should __14__ depends upon one's physical condition．Young people can safely enjoy vigorous competitive sports，but most older persons do better to limit themselves to less strenuous activities．Walking，swimming，skating are among the sports that one can enjoy and safely participate __15__ throughout life．Regularity is important if one is to get the most enjoyment and benefit out of exercise．
