如图所示,AB板长为L,B端放有质量为m的小物体P(可视为质点),物体与板的动摩擦因数为μ,开始时板水平,若绕A缓慢转过一个小角度a的过程中,物体始终保持与板相对静止,则这个过程中() A. 重力对P做正功 B. 摩擦力对P做正功 C. 合外力对P做正功 D. 支持力对P做正功Mr Li is a kind teacher. He devotes most of his time _______ us with our lessons.A. to help B. to helping C. help D. helping