avenus wrote: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

rahulg83 wrote:Nothing much...
Both provide some attribute to a certain subject.
similar type of idioms, but generally so X as to Y is not preferable now, and GMAT avoids it as per my information.
eg. He is so smart as to be brilliant.