OG详解-OG7 文法 Q6

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”).
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 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 is incorrect. Without a comma preceding it, the conjunction “and” can’t be used in this way to join sentences.