问题
Many air flights _________because of the eruption of the volcano, in _______case, many passengers had no choice but to take a train
A:had been cancelled; which
B:have been cancelled; what
C:had been canceling; that
D:have been canceling; where
B:have been cancelled; what
C:had been canceling; that
D:have been canceling; where
正确答案是：
