what time will the next train leave

题目

what time will the next train leave
这句话:的回答以下两种都可以吗?
It's 8:19
At 8:19
书上是用AT 8:19.
为什么不能用第一个呢?

答案

因为what time will the next train leave 的完整回答是the next train will leave at 8:19.所以简略为at 8:19.