Unlike a hurricane, which can be observed from withn, 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 impratical C. for such studies as this ti have been impractical D. as to not make such a study practical E. as to be impractical of study 不明白so as to的用法
the best candidate seems to be A, "so as to...:" choices are not considered by the uncorrected subjective. B compared to A is awkward by using "it" and "that" which can be avoided to make better clarity.