People ____ do exercise every day have a better chance of staying healthy than those _____don't.

[      ]

A. who; /

B. /; who

C. who; who

D. /; /

根据砂砾质土与粘性土失水压密机制，试分析典型山前洪积扇各地段（扇顶、中部和扇前缘）地面沉降的特点、危害及其原因？