A：whether light goes faster than sound
B：how many colors were there in a rainbow
C：that the moon travels around the earth
D：what would the world be like in the future
——Don't open the door, _______who knocks on it.——No problem, mom. You've told me many times.
小明的父母都是环卫工人，他很怕同学知道后瞧不起他。对此，你应该告诉他( )
