Most young people enjoy some form of physical activity. It may be walking, cycling or swimming, in winter, or skating or skiing. It may be a of some kind ― football, hockey, golf or tennis. It may be mountaineering. These who have a passion 【26】 climbing high and difficult mountains are often 【27】 with astonishment. Why are men and women 【28】 to suffer cold and hardship, and to 【29】 on high mountains This astonishment is caused, probably, by the difference between mountaineering and other forms of activity 【30】 which men give their leisure. Mountaineering is a sport and not a . There are no man-made roles, as there are for 【31】 s as golf and football. There are, of course, rules of different kinds which it would be erous to 【32】 ,but it is this from man-made rules 【33】 makes mountaineering attractive to many people. Those who climb mountains are free to use their own 【34】 If we 【35】 mountaineering with other more familiar sports, we might think that one big difference is 【36】 mountaineering is not a" team work". We should be mistaken in this. There are, it is true, no "matches" 【37】 "teams" of climbers, but when climbers are on a rock face linked by a rope on which their lives may 【38】 ,there is obviously teamwork. A mountain climber knows that he may have to fight forces that are stronger and more powerful than man. He has to fight 【39】 of nature. His sport requires high mental and 【40】 qualities. A mountain climber 【41】 to improve in skill year after year. A skier is probably past his best by the age of thirty, and most international tennis champions 【42】 in their early twenties. But it is not 【43】 for men of fifty or sixty to climb the highest mountains in the Alps. They may take more 【44】 than younger men, but they probably climb with more skill and less 【45】 of effort, and they certainly experience equal enjoyment.

A．which
B．that
C．how
D．why