Houseflies that hatch in summer live only about three weeks, but those

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.

(A) weeks, but those that emerge in the cooler days of fall often --- Answer choice.
(B) weeks, but those that emerge in the cooler days of fall often live as long or -- The positive degree of comparison is incomplete.
(C) weeks, which is different from those that emerge in the cooler days of fall and often live -- The relative pronoun 'which' has no logical referent.
(D) weeks; then those that emerge in the cooler days of fall often live as long as or -- We do need a contrasting conjunction here. 'Then' is not a contrast marker.
(E) weeks; this is different from those that emerge in the cooler days of fall, who often live -- 'This' is colloquial and 'who' cannot be justified.

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.

(A) weeks, but those that emerge in the cooler days of fall often --- Answer choice.
(B) weeks, but those that emerge in the cooler days of fall often live as long or -- The positive degree of comparison is incomplete.
(C) weeks, which is different from those that emerge in the cooler days of fall and often live -- The relative pronoun 'which' has no logical referent.
(D) weeks; then those that emerge in the cooler days of fall often live as long as or -- We do need a contrasting conjunction here. 'Then' is not a contrast marker.
(E) weeks; this is different from those that emerge in the cooler days of fall, who often live -- 'This' is colloquial and 'who' cannot be justified.

(A) weeks, but those that emerge in the cooler days of fall often live - right choice
(B) weeks, but those that emerge in the cooler days of fall often live as long or - it should be "as long as", but we do not see any
(C) weeks, which is different from those that emerge in the cooler days of fall and often live - "which" wrongly modifies weeks
(D) weeks; then those that emerge in the cooler days of fall often live as long as or - "as long" is wrong again, in addition to "then" wrong conveys the sequential meaning.
(E) weeks; this is different from those that emerge in the cooler days of fall, who often live - "this" must replace a noun, but we do not see any noun here that logically replaces "this"

nice explanation.

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.

Sentence Structure:
Houseflies (Subject)
that hatch in summer (Relative clause modifying the subject "houseflies")
live (Verb)
only about three weeks (more information about houseflies)
, but (,FANBOYS - used to separate two ICs)
those (relative pronoun referring to plural "houseflies") (Subject)
that emerge in the cooler days of fall (relative clause modifying "those")
often live (Verb)
longer than six months. (more information about houseflies)

(A) weeks, but those that emerge in the cooler days of fall often live - Correct
(B) weeks, but those that emerge in the cooler days of fall often live as long or - the correct idiom is "as long as"
(C) weeks, which is different from those that emerge in the cooler days of fall and often live - which cannot refer to houseflies. We don't need the "and" out here
(D) weeks; then those that emerge in the cooler days of fall often live as long as or - we cannot start a DC after a semicolon
(E) weeks; this is different from those that emerge in the cooler days of fall, who often live - We need a noun after a demonstrative pronoun. I don't think the placement of "who" is correct either. It should be placed closer to the noun. Besides. "who" can ONLY refer to people

Correct: (A)

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