如图所示，虚线为静电场中的等势面1、2、3、4，相邻的等势面之间的电势差相等，其中等势面3的电势为0 V。一带正电的点电荷在静电力的作用下运动，经过a、b点时的动能分别为26 eV和5 eV。当点电荷运动到某一位置，其电势能变为－8 eV时，此时它的动能应为（      ）

    We are learning English, but how can we learn English well ? A student can know a lot about English, but maybe he can't speak English.
If you want to know how to swim, you must get into the river. And if you want to be a football player, you must play football. So, you see, you can learn by using it. You must listen to your teacher in class. You must speak English to your classmates every day and also you must write something in English. Then one day you may find your English very good .

关于氨基酸的叙述错误的是（　　）

如图，小明在大楼45米高（即PH=45米，且PH⊥HC）的窗口P处进行观测，测得山坡上A处的俯角为15°，山脚B处得俯角为60°，已知该山坡的坡度i（即tan∠ABC）为1： $\sqrt{3}$ ．（点P、H、B、C、A在同一个平面上．点H、B、C在同一条直线上）