The answer is A. “Researchers' estimates of the actual number vary from three hundred to six thousand, but everyone knows that ads are everywhere.” (paragraph 1)
Step-by-step explanation:
Conjunctions link grammatical elements. In the case of the coordinating conjunctions, which includes and, but, or, yet, etc., these are mainly used to coordinate or join elements of the same rank. This applies to complete ideas known as independent clauses, which creates compound sentences.
This can be seen in sentence A because this contains the coordinating conjunction "but". Also, you can know this is a coordinating conjunction because each of the clauses "but" joins is a complete idea or independent clause.