SAT English Practice Question

The researchers’ experiments showed that, in contrast to earlier studies, the brain regions most active during sleep are not always the same in all individuals, and that this variation is related to each individual’s unique sleep cycle. Which choice best combines the sentences at the underlined portion so that it conforms to the conventions of Standard English?

1. A.the brain regions most active during sleep are not always the same in all individuals; this variation is related to each individual’s unique sleep cycle.
2. B.the brain regions most active during sleep are not always the same in all individuals, this variation is related to each individual’s unique sleep cycle.
3. C.the brain regions most active during sleep are not always the same in all individuals; and this variation is related to each individual’s unique sleep cycle.
4. D.the brain regions most active during sleep are not always the same in all individuals, and this variation is related to each individual’s unique sleep cycle.