如图所示，一束光线从水中斜射向空气，入射光线及法线已画出．请画出这束入射光线的反射光线和折射光线的大致传播方向．

如图所示的空间，匀强电场的方向竖直向下，场强为E，匀强磁场的方向水平向外，磁感应强度为B，有两个带电小球A和B都能在垂直于磁场方向的同一竖直平面内做匀速圆周运动（两小球间的库仑力可忽略），运动轨迹如图中所示，已知两个带电小球A和B的质量关系为m_A=3m_B，轨道半径为R_A=3R_B．则下列说法正确的是（   ）

       American schools begin in September after a long summer holiday. There're two terms in a school year. The first term is from September to January, and the second term is from February to June. Most American children begin to go to school when they're five years old. Most students are seventeen or eighteen years old when they finish high school.
      High school students take only five or six subjects each term. They usually go to the same classes every day, and they have homework for every class. After class, they do a lot of interesting things.
     After high school, many students go to college(大学). They usually have to pay a lot of money. So, many college students work after class to get money for their studies.

在“草→蚱蜢→青蛙→蛇”这食物链中，有毒农药在体内积累量最多的生物是（　　）