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

螺旋CT的硬件结构主要包括（）。

A.探测器

B.三维重建

C.准直器

D.X线管

E.滤过器