sam usually does his lessons_____than his classmates(slow) 这句话用比较级,为什么答案是要加more

Sam usually 【does 关键词,实义动词 】his lessons_【more slowly_】__than his classmates(slow)
修饰实义动词,要用副词,而不是形容词.
表示动作状况时,常用副词slowly.
slow作副词时,【常用于复合词】 慢慢地,缓慢地.