请以资深中国物理教师的身份，根据这道题帮我讲解金属环匀速通过磁场的面积变化，要求有证明讲解过程和图像配合，要求图像规范合理，不要出现错误。---**Extraction Content:** **Question Stem:** 8. 如图所示, 纸面内有一半径为 R 的金属环 P, 其右侧有宽度为 R 垂直纸面向里的匀强磁场, 金属环 P 从左向右以速度 v 匀速穿过磁场的整个过程中, 下列说法正确的是 ( ) **Options:** A. P 中感应电流方向始终不变 B. P 中感应电流方向为前段逆时针后段顺时针 C. P 中感应电流大小只有增加、减小两个变化过程 D. P 中感应电动势最大值为 2BRv **Chart/Diagram Description:** * Type: Diagram illustrating a physical setup involving a metal ring and a magnetic field. * Main Elements: * A circle labeled "P" representing a metal ring. * A rectangular region located to the right of the circle, bounded by two vertical lines. This region represents a magnetic field. * The region is filled with 'x' symbols, indicating that the magnetic field is directed into the plane of the paper. * The magnetic field region is labeled "B" above it, indicating the magnetic field strength. * The text describes the metal ring P has a radius R and the magnetic field has a width R. * The scenario described is the ring P moving from left to right with a constant velocity v through the magnetic field.