问题补充：
As far as I know, this maths problem is that one.
A. twice so difficult as
B. twice as difficult as
C. as twice difficult as
D. so twice difficult as
答案：
A. twice so difficult as
(答案→)B. twice as difficult as
C. as twice difficult as
D. so twice difficult as
解析：考查倍数的表达法。倍数的表达倍数应放在最前边，可排除C和D，此题的结构为：倍数+ as…as，句意：据我所知，这道数学题是那道数学题的两倍难。所以选B。
