题目

处于n=4能级的大量氢原子，向低能级跃迁时，辐射光的频率有（ ）A.4种 B.6种 C.11种 D.12种 答案：【答案】B【解析】现有大量的氢原子处于的激发态，当这些氢原子向低能级跃迁时，辐射光子的频率为种。故选B。You ________ last week if you were really serious about your work. 　 A. ought to come　　 B. ought to be coming　 C. ought to have come　 D. ought have come