如图（a），直线MN表示某电场中一条电场线，a、b是线上的两点，将一带负电荷的粒子从a点处由静止释放，粒子从a运动到b过程中的v﹣t图线如图（b）所示，设a、b两点的电势分别为φ_a、φ_b，场强大小分别为E_a、E_b，粒子在a、b两点的电势能分别为W_a、W_b，不计重力，则有（   ）

纳米银由于抗菌性能良好而被广泛应用于食物容器、个人护理品等商品中，但其释放到水环境中的风险也引起了研究者的重视．用单细胞小球藻研究纳米银的毒性，开展了如图实验．请回答：

一个正方形沿边对折两次后得到____形。

根据短文内容判断下列句子的正误。

A

    Baseball isone of the most popular sports in the USA. Each team has nine players. The baseball season goes from April to September. The members of the important baseball teams become heroes (英雄). At the end of the season, the two top teams play together. Lots of baseball fans go to watch the games. There are also thousands of baseball fans listening to the radio or watching TV. People always talk about the games. They still talk about the results (结果) and the players, even if (即使) the games are over. American football is more popular than baseball in the USA. When the baseball season ends, the football season begins. When there is an important American football game, millions of people sit in front of the TV set to watchit. Many people even sit beside the radio to hear the result when they are working.