When ____to compare the education systems of China and Germa

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When ____to compare the education systems of China and German,the professor gave no comment.
a.asked him b.asked c.being asked d.asking him
请问应该选哪个请讲一下原因

选B
因为 when 后面省略了he was ,这是把时间状语从句简化为过去分词短语作状语.
语法是：
句子并不是越复杂越好,如果在一个句子中有多个从句,会使人发晕,所以应尽量简洁,简单的方法之一就是把从句改为现在分词短语（主动的动作）或过去分词短语（被动的动作）.例如：
1.When (I am) helping others,I feel happy.
2.When (I am) asked to help others,I never hesitate.
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