The shifting punctuation and the FANBOYS conjunction "but" in the choices are clues that this question is testing sentence structure. As you read the text from the beginning, be on the lookout for clauses that could be a sentence. Clauses with a subject and predicate verb that convey a complete idea are independent clauses that need to be connected properly. Here, the clause preceding the blank, "A plan . . . would link . . . ," contains a subject, predicate verb, and a complete idea, so, it is an independent clause. The clause following the blank, "some researchers predict . . . ," is also an independent clause. Predict that the correct answer will contain one of the four ways to connect these clauses: a comma together with a FANBOYS conjunction, a semicolon alone, a period, or phrasing that makes the second clause dependent on the first. Choice (D) uses a comma and the FANBOYS conjunction "but" and is correct. All the incorrect choices create run-ons.