阅读短文，回答问题。

    It's eight o'clock. The children go to school by car every day. But they are walking to school today. It's ten o'clock. Mrs. Green usually(通常) stays at home in the morning. But she is going to the shops this morning. It's nine o'clock. Mr. Green usually reads his newspaper at night(晚上). But he is reading an interesting(有趣的) book this evening.

已知关于x的方程3x+a=2的解是x=5，则a的值是（）

如图所示，空间分布着方向平行于纸面且与场区边界垂直的有界匀强电场，电场强度为E、场区宽度为L．在紧靠电场右侧的圆形区域内，分布着垂直于纸面向外的匀强磁场，磁感应强度B大小未知，圆形磁场区域半径为r．一质量为m，电荷量为q的带正电的粒子从A点由静止释放后，在M点离开电场，并沿半径方向射入磁场区域，然后从N点射出，O为圆心，∠MON=120°，粒子重力可忽略不计．

宋代东京城内的“瓦子”是（　　）