单项选择题

A mountain climber continues to improve in skill year after year. A skier is probably (28) his best by the age of thirty, and most international tennis champions are in their early twenties. But it is not (29) for men of fifty or sixty to climb the highest mountains. They may take more time than younger men but they probably climb with more skill and less (30) of effort. They certainly experience equal enjoyment.

30()

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