Houseflies that hatch in summer live only about three weeks, but those that emerge in the cooler days of fall often live longer than six months.
Nothing is wrong with A, so let's keep it.
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(B) weeks, but those that emerge in the cooler days of fall often live as long or
- proper form is "as long as", second "as" cannot be omitted.
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(C) weeks, which is different from those that emerge in the cooler days of fall and often live
- "which" touches "weeks", resulting in illogical meaning
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(D) weeks; then those that emerge in the cooler days of fall often live as long as or
- "then" could not be after ";". ';' has to connectc two independent clauses.
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(E) weeks; this is different from those that emerge in the cooler days of fall, who often live
- it is not allowed to use free-standing "this"/"these" on GMAT; they can only be used as adjectives for the following noun.