Unlike a hurricane, which can be observed from within, a tordado is so small 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A。LZM 说其它答案主语不一致。我选D，因为它看起来虽然别扭，但主语还是一
致的。我觉得A的完成时又不是很必要。为什么A对D不对呢？请指教。