SAT Question ea8f4658: Boundaries answer and explanation

Choice A is incorrect. This choice results in a punctuation error. The relative clause "which is known as Brownian motion" is a nonessential supplement. Nonessential supplements need to be set apart from the rest of the sentence with a pair of commas, dashes, or parentheses, so we can’t use a colon here. Also, notice that colons can only come after an independent clause, which isn’t the case here.

Choice C is incorrect. This choice results in a punctuation error. The relative clause "which is known as Brownian motion" is a nonessential supplement, so it should be separated from the rest of the sentence by a pair of matching punctuation marks. We already have a comma after "motion," so we need to add a comma before "which." This choice is missing that comma.

Choice D is incorrect. This choice results in a sentence fragment. "This type of random, continuous movement" is not an independent clause and can’t stand alone as a full sentence, so we can’t put a period here.