北京大前门:Why do we need sleep

Why do we need sleep Why do we need to wash our hands freuently? why do you need sunglasses? why do we study English ? why do we study English why do we learn english We need to do my home work,____ ____? We need to do my home work,____ ____? we will need -------help to do the work (add) we must buy things we ( )A. need not B.do not need C.have not need解释一下，谢谢