选 while.
when 跟while的区别：这两个词都可用作从属连词,表示时间,意思都是"当(在).的时候",但两者之间有区别:when 的含义是at or during the time that,既可用于一点时间(从句的谓语动词需用终止性动词),也可用于一段时间(从句的动词用延续性动词),从句与主句里面的谓语动词所表示的动作或状态能同时发生或一先一后分别发生.
例如：He wants to help people when they are ill.他想在人们生病时帮助他们.
When he got to Shanghai,the ship had already set off.他到达上海时,轮船已经开走了.
while 的含义是during the time that,只能用于指一段时间(从句的谓语动词必须是延续性的),从句与主句里面的谓语动词所表示的动作只能同时发生,不能一前一后发生.
例如:One day while they were working in the fields,some farmers sawsomething strange in the sky.有一天,一些农民在田间劳动的时候,看见空中有些奇怪的东西.
While I was reading,my mother was washing clothes.我读书时,我妈妈在洗衣服.
这两个词还可以用作并列连词.when 是"在那时"或"这时突然"(and just atthat time)的意思,用来连接两个并列分句,有时 when分句前有逗号把前后两个分句分开.
例如:An Arab was walking alone through the desert when he met two men.有个阿拉伯人正独自在沙漠里行走,这时他碰见了两个人.I stayed till sunset,when it began to rain.我一直呆到太阳下山,天开始下雨了.
while 意为"而","却",表示对照关系.例如：Instead,he asked his father why he was not able to hatch chickenswhile hens could.他反而问他父亲,为什么他不能孵出小鸡,而母鸡却能.