With a huge amount of food ________，all of the staff are busy preparing for the dinner tonight.

A．would serve

B．served

C．to serve

D．serving

如图，人站在自动扶梯上不动，随扶梯向上匀速运动，下列说法中正确的是(　　)

A．重力对人做负功

B．摩擦力对人做正功

C．支持力对人做负功

D．合力对人做功为零