如图所示，用质量为m、总电阻为R的导线做成单匝矩形线框MNPQ，边长PN=2d，PQ=d。该线框置于水平桌面上，线框与桌面间绝缘，它们之间的动摩擦因数为μ。在线框的右侧存在竖直方向的匀强磁场，磁感应强度为B，磁场左右边界aa′、bb′ 间的距离为d，沿aa′ 方向磁场范围足够大。在垂直MN边的水平拉力作用下，线框以速度v匀速向右穿过磁场。在运动中线框平面始终水平，且MN边与磁场的边界平行。求：

（1）MN两点间的电势差；

（2）在线框从MN边进入磁场到MN边穿出磁场的过程中，线框中感应电流产生的焦耳热Q；

（3）在线框从MN边进入磁场到PQ边穿出磁场的过程中，水平拉力对线框所做的功W。

A 13-year-old visits the school nurse because he's experiencing back pain, fatigue, and dyspnea. The nurse suspects that the child may have scoliosis. The nurse should first （）.

A．send the child home to recover.

B．inspect the child for uneven shoulder height or uneven hip height.

C．arrange for the child to have spinal X-rays as soon as possible.

D．ask the child's parent to take him to a physician immediately.