分类：教育/科学-外语学习-英语考试
问题：vegetablesarecheap___whentheyare? 
a. to buy
b.to buy them
c.to be bought
d. buying them
回答：选A.

表示主语本身的特征时,用主动表被动.
例如:The shoes sell well.鞋卖得挺好.