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The antecedent of 'it' is fairly ambiguous in C. What does 'it' represent - insulin or isolation of insulin ? Do not use your common sense to assume it as isolation of insulin. Try to answer the question as if you know nothing about insulin. On GMAT, it has to be clearly mentioned, and pronoun with an ambiguous antecedent is a straightforward killer.
vailad, the mainpoint here is the isolation of insulin..and not insulin itself... the choice which I suppose you selected (D) mistakenly refers to insulin itself in the second phrase of the sentence..which renders it incorrect.
In option (C) as per your concern, "it" is not ambigous..it correctly refers to "isolation of insulin" introduced in the first phrase...There is no mention of insulin independently hence "it" refering to just insulin does not come...
I chose (C)...however it was tough between C and D.
That was my point. I could now think of rejecting D. D would have been perfect, had it said - "discovery of insulin", "synthesis of insulin" in the second part. (It again brings 'logic' in picture, which should not be the case though)
My concern for ambiguous antecedent in C still remains. But once D out, C is the choice.