大全T889 so as to VS so that

24. Unlike a hurricane, which can be observed from within, a tornado is so small that such a study has not been practical.
(A) that such a study has not been practical
(B) that studying it that way has not been impractical
(C) for such studies as this to have been impractical
(D) as to not make such a study practical
(E) as to be impractical of study
这里的A选项用了has been； E选项so as to be impractical of study,虽然of在这里看上去不如用so as to be impractical to study, 但是感觉E选项还是个更简洁的选项。
可否请NN们帮忙分析a为什么是更好的选项？谢谢

逻辑搭配问题，到底是study - impractical还是tornado - inpractical?显然是前者；此外，A中的完成时表示的是从过去到现在，但却不暗示将来会怎样，而E无法表达类似时态，所以E错；另：我没有查到任何be impractical of结构。