一道语法题

 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

  答案是A，我觉得E貌似比A还简洁，不知道E错在那里？ 望NN指教

e, the logical subject of "impratical" is tornado. Well, logically, is tornado impratical or is study impratical?

another point, i can't understand the structure "impractical of study"

open to discuss