详细解释一下这道题，把变化的过程准确地表达出来---Extraction: **Question Number:** 2. **Question Stem:** 如图,用两根等长的细绳将一匀质圆柱体悬挂在竖直木板的 P 点,将木板以底边 MN 为轴向后方缓慢转动直至水平,绳与木板之间的夹角保持不变,忽略圆柱体与木板之间的摩擦,在转动过程中 ( ) **Options:** A. 圆柱体对木板的压力逐渐增大 B. 圆柱体对木板的压力先增大后减小 C. 两根细绳上的拉力均先增大后减小 D. 两根细绳对圆柱体拉力的合力保持不变 **Diagram Description:** * **Type:** A perspective diagram illustrating a physical setup. * **Main Elements:** * **Wooden Board:** A rectangular plane, initially vertical, rotating backward about its bottom edge. * **Axis of Rotation:** The bottom edge of the board, labeled MN. * **Suspension Point:** A point P marked on the upper part of the board. * **Strings:** Two lines of apparently equal length extending from P, supporting a cylindrical body below. * **Cylindrical Body:** A cylinder suspended by the two strings. * **Rotation Indicator:** A curved arrow near the top right shows the direction of rotation (backward). * **Final Position:** Dashed lines show the horizontal position of the board after rotation. * **Labels:** P, M, N. **Constraint mentioned in the text:** The angle between the strings and the wooden board remains unchanged. **Assumption:** Friction between the cylindrical body and the wooden board is neglected.