Why are Rome’s famous concrete structures, such as the Colosseum, still standing after 2,000-plus years, when modern
concrete may not even last for ﬁfty? Scientists ______ that the secret to Roman concrete’s durability was its unique blend of
ingredients, which included volcanic ash and seawater.
explain
having explained
explaining
to explain
Choice A is the best answer. The convention being tested is the use of verb forms within a sentence. The main clause of a
sentence requires a ﬁnite (tensed) verb, and this choice supplies the ﬁnite present tense verb “explain” to indicate what the
scientists do.
Choice B is incorrect because it results in an ungrammatical sentence. The nonﬁnite participle “having explained” doesn’t
supply the main clause with a ﬁnite verb. Choice C is incorrect because it results in an ungrammatical sentence. The
nonﬁnite participle “explaining” doesn’t supply the main clause with a ﬁnite verb. Choice D is incorrect because it results in
an ungrammatical sentence. The nonﬁnite to-inﬁnitive “to explain” doesn’t supply the main clause with a ﬁnite verb.