阅读下面的短文，并根据短文后的要求答题。（请注意问题后的字数要求） If you ears are working well, you hear sounds all the time. You can't switch（关掉）your ears off, even when you are asleep.
Listening
When you really want to listen to something, you tell brain to focus on that sound. If your ears are healthy and are both working well, they can turn down other sounds while you focus on what you want to hear. If one ear isn't working __________ the other, they can't do this as well. If you hearing cells（细胞）have been damaged by too much loud noise or music, what you want to hear can get mixed up and lost in the other sounds around. With healthy ears you can choose what you want to listen to, but you can't choose what you hear.
Understanding
Your brain deals with the received sounds and tells you what they mean. Some people have problems understanding what they are hearing because their ears don't work well and they don't hear all the sounds. They brain does not get enough information to understand the messages or they may be mixed up.
Selective（选择的）hearing
Lots of mums and dads think that kids have this “problem”. They think that you select what you are going to hear and what you can't hear! Many mums say that dads do this, too. In fact, this is something we may all do to try and make our lives a bit easier. Some children wrote what they liked to hear and what they didn't like to hear.
