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889. 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
between A and D which one is best. are there any rules for usage of "so that" and "so as to ".