Boundaries (Punctuation)How the SAT tests it — and how to beat it

Choice A is the best answer. The convention being tested is punctuation between a subject and a verb. When a subject ('Thousands of marine biologists and ocean researchers') is immediately followed by a verb ('have devoted'), no punctuation is needed. Choice B is incorrect because no punctuation is needed between the subject and the verb. Choice C is incorrect because no punctuation is needed between the subject and the verb. Choice D is incorrect because no punctuation is needed between the subject and the verb.

Choice B is the best answer. The convention being tested is punctuation use between two supplementary phrases following the coordinate clause ('but she…physics'). This choice correctly uses a comma to mark the boundary between the supplementary noun phrase ('the application of physics to medicine') that defines 'medical physics' and the supplementary participial phrase ('developing…battlefield') that provides additional information about Curie's dedication. Choice A is incorrect because a semicolon can't be used to join two supplementary phrases following a coordinate clause. Choice D is incorrect because it results in a rhetorically unacceptable sentence fragment beginning with 'developing.' Choice C is incorrect because the lack of punctuation illogically suggests that the application of physics to medicine is developing mobile X-ray units.