题目内容：

He is able to axplain the ______(原因) because he was there at that time.

优质解答

He is able to axplain the _reason_(原因) because he was there at that time.

追问：

为什么不加s

追答：

没有明确说明单复数，加不加s都可以

追问：

为什么我加了s算错

追答：

不清楚，按照我的理解加不加都没有问题。。。