—Why not come and join us in the game, Linda? 

—__________, but I have something to discuss with Mr Smith at his office now.[ ]

A. Let's go

B. I'd like to

C. Yes, please

如图所示，在光滑的水平桌面上有一物体A，通过绳子与物体B相连，假设绳子的质量以及绳子与定滑轮之间的摩擦力都可以忽略不计，绳子不可伸长．如果m_B=m_A，则物体A的加速度大小等于 (    )

A．3g

B．g

C．3g/4

D．g/2