The shifting punctuation in the choices indicate 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 first sentence introduces the LIGO observatory. The clause preceding the blank, "A new technology has changed this," contains a subject, predicate verb, and a complete idea, so, it is an independent clause. The clause following the blank, "the noise is . . . reduced . . . ," 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.
Eliminate (A) and (C) since these choices create run-ons. To choose between (B) and (D), consider the logic of the text to determine if "though" belongs to the first clause (D) or the second (B). Since the description of "squeezing" the light describes the "new technology," (D) correctly separates the independent clause from "though" and the colon introduces the explanation. Choice (B) contrasts the description with the "new technology", which is not the logic of the sentence.
