OG详解-OG10 文法 Q2

Choice D is the best answer. The convention being tested is end-of-sentence punctuation. This choice correctly uses a question mark to punctuate the interrogative clause “could the blueberries thrive,” which asks a direct question at the end of the sentence. 
Choice A is incorrect because a period can’t be used in this way to punctuate an interrogative clause, such as “could the blueberries thrive,” at the end of a sentence. Choice B is incorrect because the context requires an interrogative clause. The declarative clause “the blueberries could thrive” incorrectly indicates that it was known that the blueberries could thrive in alkaline soil, whereas Michel had yet to find this out. Choice C is incorrect because a question mark can’t be used in this way to punctuate a declarative clause, such as “the blueberries could thrive,” at the end of a sentence.