The revision that uses an appositive to combine these two sentences is C. The sun is a star, which is at the center, of Earth's solar system.
It should be noted that an appositive simply means a noun or pronoun that is set beside another noun or pronoun to explain it.
In this case, the revision that uses an appositive to combine these two sentences is that the sun is a star, which is at the center, of Earth's solar system.
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