He will be back after two hours改错,答案是把after改成in,为什么不改成for、by或者during?

He will be back after two hours
改错,答案是把after改成in,为什么不改成for、by或者during?

in 指在.期间,或在.之内.一般用在将来要发生的事.
He will be back in two hours.
for 表示所用时间有多长.
He stayed at home for two hours.
during 表示在.期间,一般用在已经发生的事.
He had read 3 books during the last two days.

将来时的时间状语表达是 in＋一段时间 “......以后”