He made a(n) ______ by collecting waste materials and bought several houses in Suzhou.[ ]

A. living  

B. money

C. acquaintance  

D. fortune

如图所示，轻弹簧k一端与墙相连，处于自然状态，质量为4 kg的木块沿光滑的水平面以5m/s的速度运动并开始挤压弹簧.求弹簧的最大弹性势能及木块被弹回速度增大到3m/s时弹簧的弹性势能.