已经有几个答案被提出来来回答这个基本问题，但是没有一个被证明——甚至对它们的充分的检验也被证明是一个可怕的挑战。
Several answers have been proposed to address this fundamental question, but none has been proven-and even adequately testing them has proven to be a formidable challenge.
如果旅馆有提供游泳池的话，去游上几个来回(所以记得带上你的泳衣)？
Do laps in the hotel pool if there is one (always pack your swimwear)?
有几个男孩在擦课桌和椅子，并来回搬动。
A few boys are wiping the desks and chairs and moving them around.
有几个男孩在擦课桌和椅子，并来回搬动。
A few boys are wiping the desks and chairs and moving them around.
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