题目

Wang Xiaojun is l2 years old．He lives in the country in Hubei Province．Today is his birthday．His parents prepare a nice dinner for him, but the boy says, “I won’t have the dinner because after the dinner, today is over, and parents are leaving for Shenzhen tomorrow．I don’t want to let them go．” It is the first birthday that Wang Xiaojun is spending with his parents in three years．In the past three years, his parents stayed in Shenzhen and worked to make money．They haven’t seen their son for three years．Tomorrow they have to leave home to go to Shenzhen． Wang is not 如图所示，平行金属板内有一匀强电场，一带电粒子以平行于金属板的初速从板间某点射入电场，不计粒子受的重力，当入射动能为Ek时，此带电粒子从场中射出的动能恰好为2Ek。如果入射速度方向不变，初动能变为2Ek，那么带电粒子射出的动能为A．B．C．D．3Ek