Which choice completes the text so that it conforms to the conventions of Standard English? 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.

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.