potatoes turn green when exposed to light.exposed是过去式还是过去分词 为什么要加d
问题描述：
potatoes turn green when exposed to light.exposed是过去式还是过去分词 为什么要加d
答
过去分词
这是一种省略的结构
when (they are) exposed to light
they are 被省略,记住即可如果是主语+过去式怎么省略when doing