Chọn đáp án đúng và giải thích: Mr.Grunfeld______ for his humour already, but on this occasion the whole class_____ hysterically

Mr.Grunfeld______ for his humour already, but on this occasion the whole class_____ hysterically.
A. knew - had been laughing.
B.was known - laugh
C. Has known - laughed
D. Is known - was laughing
Vì sao chọn D???