The result of the study indicated that it was the type of fat ____ made the difference. [ ]

A. that

B. what

C. who

D. as

如图所示，重为100N的物体在水平面上向右运动，物体与水平面的动摩擦系数为0．2，与此同时物体受到一个水平向左的力F=20N，那么物体受到的合力为（）

A．0

B．40N，水平向左

C．20N，水平向右

D．20N，水平向左