So difficult that no one could a good solution to it.
A. was the problem; e up with
B. the problem was; e up with
C. the problem was; put up with
D. was the problem; put up with
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(答案→)A. was the problem; e up with
B. the problem was; e up with
C. the problem was; put up with
D. was the problem; put up with
解析：考查倒装句和区分动词短语。句义：那个问题那么难以至于没有人可以提出解决方案。把握句首关键词so ,句型so…that …如此。。。以至于。。。，如果so 位于句首构成倒装，所以排除B C ,e up with提出 put up with 忍受，根据句意，所以A正确。