My room is a mess, but I __________clean it before I go out tonight. I can do it in the morning.
A:daren't
B:shouldn't
C:needn't
D:mustn't
如图所示为氢原子能级的示意图,现有大量的氢原子处于n=4的激发态,当向低能级跃迁时辐射出若干不同频率的光.关于这些光,下列说法正确的是( )
下列做法有利于建立良好师生关系的是( )