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A virgin pine forest covers a valley, and one of the inhabitants is a red squirrel species. A large volcanic eruption occurs and separates the valley with a huge lava flow that the squirrels cannot cross, thus producing two isolated populations of squirrels. What change needs to occur for the two populations of squirrels to become separate species?  (A) The area that the squirrel populations occupy changes so that the squirrel populations exist in distinct habitats. (B) The fur color of the two squirrel populations must become distinct. (C) The two squirrel populations select mates using the same sexual behavior. (D) The two squirrel populations must eat distinct species of plants. (E) The two squirrel populations become reproductively isolated.