【问】 The population density per square mile in the United States is twenty-two people，whilethat in Canada is two people.

A. whereas B. when

C. however D. and(TOEFL，84—85)

句中的while是否可用when来代替?

【答】 while与when用法上的一个重要区别，就是while可用于表示对比(whereas)，而when却没有这个用法。

1. Jane was dressed in brown while Mary was dressed in blue. (ALD)

简穿褐色衣服，而玛丽却穿兰色衣服。

2. While their country has plenty of oil，ours has none. (LDCE)

他们国家有充足的石油，而我们国家却没有。