单选题 Unlike competitive (running), race walkers must always keep some portion of their feet in contact with the ground.无

【正确答案】 C

【答案解析】句意：跟赛跑者不一样，竞走者必须始终使脚的某一部分接触地面。这句话中相比较的是赛跑者与竞走者，而且都是复数。