C 副词　 word that adds more information about place, time, circumstance, manner, cause, degree, etc. to a verb, an adjective, a phrase or another adverb

　adverb　作“副词”解，指用来说明某事物怎么样、什么时候或在哪里的词，也用于对动词、形容词、短语或另一副词增加地点、时间、环境、方式、原因、程度等信息的词。

　relative ～　关系副词/p>
　interrogative ～　疑问副词/p>

In the sentence"Please speak slowly"，"slowly" is an adverb.在Please speak slowly这句话里， slowly是副词。

［词源］拉丁语adverbium（加到动词上的词）