-- Lisa was seen ______ an old man go across the street this morning.

--What a kind girl she is!

A．helping

B．helped

C．to help

D．helps

如图所示，小孩坐在跷跷板的一端，大象用脚踩另一端，小孩被弹起. 请画小孩在向上运动的过程中所受力的示意图. (不考虑空气阻力)