so...as to和so....that究竟哪个更优先被选

Unlike a hurricane,which can be observed from within,a tornado is so small __________.

A.that such a study has not been practical

E.as to be impractical of study

选A,      E为何错，最好能讲讲so...as to和so....that分别在那种情况下更优先被选，THX!!

so that 对于句子前后逻辑主语没要求

so as to要求前后句子逻辑主语必须一致

前面句子逻辑主语是hurricane 后面句子的逻辑主语是 tornado 只能用so that

若说的不对请高手指教