In ancient Roman architecture, builders often used _______ to create durable structures before concrete was fully refined. Volcanic ash was a key ingredient in their mortar.
Which choice completes the text so that it conforms to the conventions of Standard English?
tufa—a variety of porous limestone
tufa—a variety of porous limestone,
tufa a variety of porous limestone
tufa, a variety of porous limestone
Correct Answer: A
Choice A is the best answer. The phrase "a variety of porous limestone" defines "tufa." A single dash is often used to introduce an explanation or definition at the end of a clause or to emphasize an appositive.
Choice B is incorrect because the comma after "limestone" is unnecessary unless the sentence continued. Choice C is incorrect because it lacks punctuation to separate the term from its definition. Choice D is incorrect because while a comma could work, a dash is often stylistically preferred for emphasis or definitions, and option A provides a clean break.