OG详解-OG3 文法 Q2

Choice A is the best answer. The convention being tested is the coordination of clauses within a sentence. This choice correctly uses a comma and the coordinating conjunction “but” to join a main clause (“Typically…value”) and a subordinate clause (“when…Whitman”) that precedes a main clause (“such… scholars”). 
Choice B is incorrect because it results in a run-on sentence. A main clause is fused without punctuation and/or a conjunction to a subordinate clause that precedes a main clause. 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 a main clause and a subordinate clause that precedes a main clause. Choice D is incorrect. Without a comma preceding it, the conjunction “but” can’t be used in this way to join a main clause and a subordinate clause that precedes a main clause.