Materials scientist Marie-Agathe Charpagne and her colleagues believed they could improve on the multicomponent alloy
NiCoCr, an equal-proportions mixture of nickel (Ni), cobalt (Co), and chromium (Cr), by replacing chromium with ruthenium
______ the alloy that resulted, NiCoRu, turned out to be an unsuitable replacement for NiCoCr.
(Ru)
(Ru) but
(Ru),
(Ru), but
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.