OG详解-OG3 文法 Q20

Choice D is the best answer. The convention being tested is the coordination of main clauses. This choice correctly uses a comma and the coordinating conjunction “but” to join the first main clause (“Materials… Ru”) and the second main clause (“the alloy…NiCoCr”). 
Choice A is incorrect because it results in a run-on sentence. The two main clauses are fused without punctuation and/or a conjunction. Choice B is incorrect because when coordinating two longer main clauses such as these, it’s conventional to use a comma before the coordinating conjunction. Choice C is incorrect because it results in a comma splice. Without a conjunction following it, a comma can’t be used in this way to join two main clauses.