Everyone wants to reach the top of the mountain, but all the happiness happens while you _____ it.

A  climb     B are climbing

选择B进行体是正确的，甚至更佳，更能突出climb的延续性。但是A答案climb 的一般体也应该正确，如：I went swimming while the others played tennis.同于I went swimming while the others are playing tennis.

我觉得该练习题设置不太科学，或者说置语言实际运用于不顾。期盼专家老师权威详尽指导为学生解惑，十分感谢。