It is a serious novel, not ___________.
His teacher is the only one ______ he can turn when he is in trouble.
_____ of their bad habits, the boys switched from being dependent on their parents to _____ for others.
Happy birthday, Alice! So you have _____ twenty-one already.
The computer was used in teaching. As a result, not only _____, but students became more interested in the lessons.
Teachers have to constantly update their knowledge in order to maintain their professional(专业的)_______.