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

问题描述:

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

C for which =====why为何,之所以