【题目】Potatoes turn green when ________ to light.
A.exposed
B.are exposed
C.being exposed
D.to be exposed

【答案】A
【解析】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。