题目

      When I told my father that I was moving to Des Moines,Iowa, he told me about the only time he had been there. It was in the 1930s, when he was an editor of the literary magazine of Southern Methodist University（SMU）in Dallas, Texas. He also worked as a professor at SMU, and there was a girl student in his class who suffered from a serious back disease. She couldn’t afford the operation because her family was poor.       Her mother ran a boardinghouse in Galveston, a seaside town near Houston, Texas. She was cleaning out the attic（阁楼）one day when she came ac如图所示，水平地面上的物体受重力G和水平作用力F，物体保持静止．现在使作用力F保持大小不变，方向沿逆时针方向缓缓转过，而物体始终保持静止，则在这个过程中，物休对地面的正压力N的大小和地面给物体的摩擦力f的大小的变化情况是（  ）A．f不变B．f先变小后变大C．N先变小后变大D．N先变大后变小