请问一句话！

我也来回答一下吧,虽然我是读中文的.

The distance that light travels in one second is 300,000 kilometers.

这里是一个定语从句。

that light travels in one second修饰the distance.

that 是作travels的宾语，而that 是代名词the distance的。

所以这一道题可以用which来代替that.

判断定语从句与同位语从句最佳办法：

看which/that在句中是充当句子成分，如果充当宾语或主语，那么它一定是定语从句。

如，
He is the boy that I am after.“他是我追求的男孩。”
很显然，that在句中作after的宾语，介宾短语be after sb./sth“追求某人/某事”

He told me the good news that his team had won.“他告诉我他的队赢了。”
很显然，that在his team had won句中不充当任何成分。因为his team had won已经是一个完整的句子了。所以，这一句是一个同位语从句。that代整个句子。