Which choice completes the text so that it conforms to the conventions of Standard English?
A
vapor. With
B
vapor with
C
vapor, with
D
vapor and with
Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is punctuation use between sentences. In this choice, the period is used correctly to mark the boundary between one sentence ("What…vapor") and another ("With…point").
Why the other choices are wrong
Choice B
Choice B is incorrect because it results in a run-on sentence. The sentences ("What…vapor" and "with…point") are fused without punctuation and/or a conjunction.
Choice C
Choice C is incorrect because it results in a comma splice. A comma can’t be used in this way to mark the boundary between sentences.
Choice D
Choice D is incorrect. Without a comma preceding it, the conjunction "and" can’t be used in this way to join sentences.