When travelling, you should take care of your health.
如何分析上句中的 when travelling?
像 when travelling 这样的从属连词 + v-ing/v-ed 的结构一般可以看作是省略了主语和 be 的某种形式的时间状语从句。这个被省略的主语就是句子的主语：
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Don't reach sideways while standing on a ladder. (while standing = while you are standing) 站在梯子上(的时候), 不要侧着身子去够东西。
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He wrote his greatest novel while working on a freighter. (while working = while he was working) 当他在货船上工作时,他写出了他最伟大的小说。
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Be careful when crossing the street. (crossing the street = when you are crossing the street) 过马路时要小心。
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Nobody bought the product, however, because when translated it meant “X puts living things into dry hair.” 然而，谁也不买这个产品，因为原句翻译出来，意思是“X使干发生虫”。
除了 when 和 while 之外, 下列从属连词都能引导 v-ing/v-ed/adj: as soon as, once, although, though, as if (as though), if, even if, unless, whether ... or (参见相应的词条)。请做下面的英语试题：
____ with the size of the whole earth, the highest mountain does not seem high at all. A. When compared B. Compare C. While comparing D. Comparing(全国四级,90.1-47)