这句话要分开看.It takes the Earth the length of time to revolve once around the Sun.it是形式主语,代表a solar day简单来说,会有这样的句子：It takes me an hour to finish my homework.就是这样的句子.这句话...谢谢你的回答,我的疑问是which的用法,这句话可以用which吗?为什么? 里面是不是省略了that?否则就有2个谓语了.不可以。which的话是指代the length of time这样the length of time就变成了主语。花时间的是earth而不是the length of time谢谢,那么用that为什么不是指代前面的the length of time呢 又不明白了,可否解释下2种用法的不同.that也是指代前面的the length of time或者这样A solar day is the length of time that it takes the Earth to revolve once around the Sun.