across the railway, a train hit it.A. While walkingB. Walking slowlyC. As the little pig

across the railway, a train hit it.
A. While walking
B. Walking slowly
C. As the little pig walking
D. While the little pig was walking

A. While walkingB. Walking slowlyC. As the little pig walking(答案→)D. While the little pig was walking解析：考查句子结构：逗号后面是简单句，主语是a train，A项用的是状语从句的省略式，只有主语一致的时候，才能省略从句的主语和be动词，但是这两句话的主语不一致，A是错的，如果用非谓语作状语的时候，应该非谓语的逻辑主语是a train，而B项walking的逻辑主语是人，所以也不对，C用的是as引导的时间状语从句，但是句子缺少be动词，所以选D。句意：当小猪穿越铁轨的时候，一辆火车撞到它。选D。

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