The Alvarez theory, developed in 1980 by physicist Luis Walter Alvarez and his geologist son Walter Alvarez, maintained that
the secondary effects of an asteroid impact caused many dinosaurs and other animals to die ______ it left unexplored the
question of whether unrelated volcanic activity might have also contributed to the mass extinctions.
out but
out, but
out
out,
Choice B is the best answer. The convention being tested is the coordination of main clauses within a sentence. This choice
correctly uses a comma and the coordinating conjunction “but” to join the ﬁrst main clause (“the Alvarez…out”) and the
second main clause (“it left…extinctions”).
Choice A 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 run-on sentence. The two main clauses are
fused without punctuation and/or a conjunction. Choice D 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.