Split #1: good old-fashioned subject-verb agreement! The subject is the word "impasse", which is singular. A whole lot of plural stuff follows this in the modifier, but the subject is singular and requires a singular verb. The verb "were" is incorrect, and Choices (A) & (B) & (C) make this mistake. The correct verb is "was", which the other three choices have.
Split #2: the idioms with "so". The idiom "so great that it required" is perfectly
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Split #2: the idioms with "so". The idiom "so great that it required" is perfectly
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