题目

    My mother used to ask me what the most important part of the body is．Through the years I 16   what the correct answer was.When I was   17  ，I thought sound was important to us，so I said，“My  18  ”She said，“No．Many people are   19  ．But keep  20  it and I will ask you again late.”        Several years passed before she asked me again．This time I told her，“Mommy，if there is 21  important to us．it must be our eyes．”She looked at me and said，“You are learning   22，but the answer is not correct because there are many people who 19．如图所示，真空中有一个截面半径为R，分布均匀的圆形玻璃砖，频率为v0的细激光束在真空中沿直线BC传播，并于玻璃砖表面C点经折射进入玻璃砖，且在玻璃砖表面D点又经折射进入真空中，∠COD=120°，已知玻璃对该激光的折射率为$\sqrt{3}$，则下列说法中正确的是（　　）A．图中的激光束在C处的入射角为30°B．此激光束在玻璃砖中穿越的时间t=$\frac{3R}{c}$（其中c为真空中光速）C．改变入射角α的大小，细激光可能在玻璃砖的内表面发生全反射D．无论如何改变入射角α的大小，细激光也不可能在玻璃砖的内表面发生全反射