--What did the teacher say to you just now?  ---She asked me ___.

A．if I hace finished my homework

B．if my homework had finished or not

C．whether or not had I finished my homework

D．whether I had finished my homework

如图所示，有一辆汽车满载西瓜在水平路面上匀速前进。突然发现意外情况，紧急刹车做匀减速运动，加速度大小为a，则中间一质量为m的西瓜A受到其他西瓜对它的作用力的大小是（     ）

A．

B．ma

C．

D．m(g+a)