GMAT Sentence Correction (SC) | Re: Researchers hypothesize that granitic soil is the ideal

I think you mean "so X as to Y." That can work, although it's not the clearest way to say what we want to say. C has bigger problems, though. First, "soft enough so it causes" is not idiomatic. We need "soft enough to cause" or "so soft that it causes." Even if this were okay, it wouldn't be parallel with "so hard as."

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