题目

There was once a young man who liked puppets (木偶) so much that he became an apprentice (学徒) to a master puppet-maker. Sadly, the young man was not so clever, and his teacher and the other apprentices were always telling him he had no talent for making puppets, and that he would never amount to anything. Even so, he enjoyed it so much that he worked day after day to improve. Despite his efforts, they would always find something wrong with his puppets, and the poor puppets ended up being thrown out of the workshop. The young man wasn’t going to give up, so he decided that from 如图甲所示，水平导轨abd所在空间存在有垂直导轨平面的匀强磁场，从某时刻开始计时磁场的磁感应强度随时间变化的规律如图乙所示（垂直导轨平面向上为磁场正方向）。已知导体棒PQ放置在导轨上且始终静止不动，下列说法正确的是（　　）A. 导轨一定粗糙B. t1时刻PQ中电流方向发生改变C. 导体棒PQ受安培力大小先减小后增大D. t1时刻PQ中电流为0