The correct answer among the choices is option D. The word "swiftly", which is italicized in the example, is an adverb. An adverb modifies a verb, an adjective or other adverbs. In the sentence, the verb modified is swim which is a verb.
Answer:
"Without innovations in food production, the factories could not have grown so quickly. The trend quickened after 1860 and continued throughout the 21st century as well."
Explanation:
Because in the previous sentence it already stated the same thing. (PS I probably got this wrong)