下列短文中有十个空白。每个空白有四个选项。根据上下文要求选出最佳答案。

Most young people enjoy some form of physical activity. It may be walking, bicycling, or swimming, or in winter, skating or skiing. It may be a of some (11) -football, hockey, golf or tennis. It may be mountaineering. Those who have a passion for climbing high and difficult mountains are often (12) with astonishment. Why are men and women willing to suffer cold and hardship, and to take risks on high mountainsThis astonishment is caused, (13) , by the difference between mountaineering and other forms of activity to which men give their leisure. Mountaineering is a sport and (14) a . There are no man-made rules, as there are for such s as golf and football. There, (15) , rules of a different kind which it would be erous to ignore, but it is this from man-made rules that makes mountaineering (16) to many people. Those who climb mountains are free to use their own methods. If we compare mountaineering and other more (17) sports, we might think that one big difference is that mountaineering is not a“team ”. We should be mistaken (18) this. There are, it is true, no“matches”between“team”of climbers, but when climbers are on a rock face linked by a rope on which their lives may depend, there is (19) teamwork. The mountain climber knows that he may have to fight forces that are stronger and more powerful than man. He has to fight the forces of (20) . His sport requires high mental and physical qualities.