Question 9·50 Super-Hard SAT Grammar Questions·Standard English Conventions

Start with The similarity between the stars' motions: what does this subject do? It initially seemed to support the conclusion, and the rest tells us what that conclusion was. The first part is a full sentence, which means we need to find where the next full sentence begins to choose the punctuation.

What follows the reason introduced by because? Past showed that the stars were hundreds of light-years apart, we find astronomers ultimately rejected that conclusion, with its own subject and main verb. The reason alone would be a fragment, a group of words that can’t stand alone, but the words about the astronomers complete the thought. The whole second part can stand alone, even though its opening reason cannot.

Now test the version with a semicolon but no because: what does its later comma join? The words ending hundreds of light-years apart can now stand alone, and the words saying the astronomers rejected the conclusion can too. The comma between them makes a comma splice, a comma alone joining two full sentences, and the earlier semicolon doesn’t fix it.

Which punctuation can join the first full sentence to the complete sentence ending astronomers ultimately rejected that conclusion? The version with a comma before because leaves only a comma between full sentences, while the version with no mark leaves no punctuation there. A semicolon can join two full sentences, so the semicolon before because works. Choice A.