关于so as to的一个小问题！先谢谢大家了！

上xdf的时候，赵丽说过so as to和so that 相仿，当主语一致的时候首选so as to，可是做og的时候发现37题

37.    Although schistosomiasis is not often fatal, it is so debilitating that it has become an economic drain on many developing countries.
(A)    it is so debilitating that it has become an economic
(B)    it is of such debilitation, it has become an economical
(C)    so debilitating is it as to become an economic
(D)    such is its debilitation, it becomes an economical
(E)    there is so much debilitation that it has become an economical

选A

开始想可能是倒装复杂化的原因，但是
给C的解释是    The construction so x as to y is not a correct idiom.

请大家帮忙看一下，是so as to不能用吗？但是以前做小蓝的时候碰上so that和so as to，赵丽说的都挺好用的啊？

谢谢！！