问题
So difficult _______ that no one could ______ a good solution to it.[ ]
A: was the problem; come up with
B: the problem was; come up with
C: the problem was; put up with
D: was the problem; put up with
B: the problem was; come up with
C: the problem was; put up with
D: was the problem; put up with
正确答案是：
