【题目】Potatoes turn green when ________ to light.
A. exposed B. are exposed
C. being exposed D. to be exposed
在线课程【答案】A
【解析】
试题分析：考查非谓语动词。be exposed to遭受，暴露于…，这里的exposed可以看作 when they are exposed to the light的省略。因为句子中有when,while等这些词时，从句可以省略与主句相同的主语和be动词。是使显露的意思。故选A。