She hopes that I can _____ her invitation as soon as possible so that she can make the preparations (准备) for the party.       [ ]

A. reply to                    

B. send up    

C. write down                  

D. pick up

如图所示，质量为M的平板小车放在倾角为θ的光滑斜面上（斜面固定），一质量为m的人在车上沿平板向下运动时，车恰好静止，求人的加速度