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 ______blank that the secret to Roman concrete’s durability was its unique blend of ingredients, which included volcanic ash and seawater.
Which choice completes the text so that it conforms to the conventions of Standard English?
Correct answer: A
Explanation
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.
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