题目

Bobby Moresco grew up in New York's Hell's Kitchen, a tough working-class neighborhood on Manhattan's West Side. But Hell's Kitchen lies right next door to Broadway, and the bright lights attracted Bobby from the time he was a teen. Being stage-struck was hardly what a street kid could admit to his partners. Fearing their ridicule, he told no one, not even his girlfriend, when he started taking acting lessons at age 17. If you were a kid from the neighborhood, you became a cop, construction worker, longshoreman（码头搬运工人）or criminal. Not an actor.   Moresco struggled to make tha如图所示，以水平力F压物体A，这时A沿竖直墙壁匀速下滑，若物体A与墙面间的动摩擦因数为μ，A物体的质量为m，那么A物体与墙面间的滑动摩擦力大小等于（ ）A．μmgB．mgC．FD．μF