解这道题---**Question Stem:** 如图所示，放在水平桌面上的物块用细线通过定滑轮与沙桶相连，当沙桶与沙的总质量为m时，物块恰好做匀速直线运动（忽略细线与滑轮之间的摩擦）。以下说法正确的是（ ） **Chart/Diagram Description:** * **Type:** A schematic diagram illustrating a physics setup. * **Main Elements:** * **Block:** A rectangular grey block is placed on a horizontal surface (table). * **Table:** A horizontal surface, indicated by a shaded area with a rough texture (zigzag lines often represent friction or a surface). The table has an edge over which a pulley is mounted. * **String/Thin line:** A thin line connects the block on the table to a bucket. It runs horizontally from the block, passes over a pulley, and then hangs vertically. * **Pulley:** A fixed pulley is mounted at the edge of the table. The string passes over it. * **Bucket:** A yellow bucket with a handle is suspended vertically by the string below the pulley. * **Relative Position and Direction:** The block is on the left side of the table. The string goes from the right side of the block, over the pulley at the table's edge, and down to the bucket. The table extends to the right under the pulley, indicating it's a fixed part of the setup. **Options:** A. 物块受到的滑动摩擦力大小为mg B. 物块的重力与它对桌面的压力是一对平衡力 C. 物块受到的滑动摩擦力与支持力是一对平衡力 D. 继续向沙桶中加入沙子，物块受到的滑动摩擦力增大