题目

Mr Brown was going away for a week. Before he left, he said to his son, if anyone asks for me, you can tell him that your father has been out for doing something, and will be back in a week, then be sure to ask him to sit down for a cup of tea.  OK, Dad, said his son. But he was afraid his son couldn't remember this, he wrote these words down on a piece of paper and gave it to him. His son put it into his small pocket, took it out and looked at it now and then.  Four days passed, but no one came to see his father. The boy thought that there was no man to come and that the piece of paper was 红、黄、绿三种单色光以相同的入射角到达某介质和空气的界面时,若黄光恰好发生全反射,则  (  )                                                                      A．绿光一定能发生全反射B．红光一定能发生全反射C．三种单色光相比,红光在介质中的传播速率最大D．红光在介质中的波长比它在空气中的波长长