The secret to turning a daily stroll into a meaningful form of exercise is to pick up the pace.
【正确答案】 C
【答案解析】解析：句意：把每天的漫步变成一种有意义的锻炼形式的秘诀是加快速度。pick up加快，加速。accelerate加速。dilate扩大，膨胀。multiply繁殖，增加。enlarge扩大，放大。