Bird lovers know that a good place to go bird-watching is Guandu Nature Park. As many as 220 kinds of birds come to this park every year. Since the park offers birds lots of food to eat and places to build their nests, it has become a paradise（天堂）for birds.

    Guandu Nature Park is also a great outdoor classroom. The park's Nature Center provides guided tours to the public. During the tour, visitors can learn about the environment. So, if you want to have a fun and educational weekend, come to Guandu Nature Park. Get away from city life and head into nature.

如图所示，一电荷量为q的负电荷以速度v射入匀强磁场中，其中电荷不受洛伦兹力的是（   ）

牙人，旧时居于买卖双方之间，从中撮合，以获取佣金的人。宋朝时期，土地典当买卖中，在大部分的情形下，牙人都参与交易过程，并在价格磋商中起很大作用。牙人在土地买卖中起着越来越重要的作用说明（  ）

温度升高时，温度计内的酒精柱会上升，从微观角度分析，其原因是