题目

---How long has your brother been working in this factory?    ---_____he dropped from school in 2010.    A. When      B. Until      C. Since      D. Before 答案：C5．α粒子的质量是质子质量的4倍，电荷量是质子电荷量的2倍，它们从静止起，经同一电场加速，获得的动能之比Eα：EP=2：1，获得的速度之比vα：vP=1：$\sqrt{2}$．