I know the reason___he was unhappy at the party that day.A in whi_英语解答

I know the reason___he was unhappy at the party that day.A in which B on which C for which D that
I know the reason___he was unhappy at the party that day.
A in which B on which C for which D that

人气：193 ℃　时间：2020-01-27 08:01:45

解答

C for which =====why为何,之所以