题目

________ a good breakfast, my son. It's good for your health. Mom often says to Tommy. A . Eat B . Eating C . To eat D . Eats 答案：A下图中画出了氢原子的4个能级,并注明了相应的能量E.处在n=4能级的一群氢原子向低能级跃迁时,能够发出若干种不同频率的光波,已知金属钾的逸出功为2.22 eV,能够从金属钾的表面打出光电子的光波总共有(    ) A.两种              B.三种              C.四种               D.五种