题目

阅读下面短文，根据所给情节进行续写，使之构成一个完整的故事。Gramma Goodie visits her grandkids every Sunday afternoon. Every visit starts the same. First she rings the doorbell. Then Chris jumps down the stairs. Jamie runs to open the door. And every time, Gramma Goodie has gifts. But the kids never know what she'll bring.Sometimes it's something you need, like socks or underwear. Sometimes it's movie tickets or a giant teddy bear. It could be a kite or a stuffed (填充的) baby kangaroo. It could be a lollipop (棒棒糖) that turns your tongue blue.Well, one Su如图所示，光滑平行金属导轨MN、PQ所在平面与水平面成θ角，M、P之间接一个阻值为R的定值电阻，阻值为r的金属棒ab垂直导轨放置且与导轨接触良好，其他部分电阻不计。整个装置处在磁感应强度为B的匀强磁场中，磁场方向垂直于导轨平面向上。t＝0时对金属棒施加一个平行于导轨的外力F，棒由静止开始沿导轨向上运动，通过R的电荷量q随时间的二次方t2的变化关系如图乙所示。图中关于穿过回路abPMa的磁通量Φ、金属棒ab加速度a、金属棒受到的外力F、通过棒的电流I随时间t变化的图像正确的是(　　)A. B. C. D.