The answer is A because an adverb phrase is two or more words that act as an adverb. An adverb is an adjective that describes the verb. In this case, the adverb would be "slowly."
<span> are a base 10 system, but they're not a </span>place-value<span> system. Until the Indo-Arabic system became the standard written way to denote numbers, most of the </span>numeral<span> systems of the world were some sort of base 10 system, but not a</span>place-value<span> system.</span>
