题目

    We walked in so quietly that the nurse at the desk didn’t even lift her eyes from the book. Mum pointed at a big chair by the door and I knew she wanted me to sit down. While I watched mouth open in surprise, Mum took off her hat and coat and gave them to me to hold. She pushed the mop past the desk and as the nurse looked up; Mum nodded and said, “Very dirty floors.” “Yes, I’m glad they’ve finally decided to clean them,” the nurse answered. She looked at Mum strangely and said, “But aren’t you working late?” Mum just pushed harder each sweep of the mop taking her fa 一端装有定滑轮的粗糙斜面体放在地面上，A、B两物体通过细绳连接，并处于静止状态(不计绳的质量和绳与滑轮间的摩擦)，如图所示.现用水平力F作用于物体B上，缓慢拉开一小角度，此过程中斜面体与物体A仍然静止.则下列说法正确的是 (　　)A．水平力F不变                    B．物体A所受斜面体的摩擦力一定变大C．物体A所受斜面体的作用力不变     D．斜面体所受地面的支持力一定不变