阅读下面短文，按照句子结构的语法性和上下文连贯的要求，在空格处

填入一个适当的词或使用括号中词语的正确形式填空。

    All ____ (student)need to have good habits：When you have good study habits, you learn things ____ (quick). You also remember____ (they) easily.

    Do you like to study in the ____ (live) room? This is not a good place ____ it is usually too noisy. You need to study ____ a quiet place，like your bedroom. A quiet place will help you only ____ (thought)about one thing，and you will learn better.

     Before you begin to study, don't forget ____ (clean) your desk. A good desk light is ____ (importance), too. You'll feel tired easily if there ____ (be) not enough light.

“姑苏城外寒山寺，夜半钟声到客船”，这是唐诗中的名句，船上的客人听到寒山寺的钟声，这是由于寒山寺的大钟受撞击发生了____，在____中以____形式传入人的耳中，引起人耳的____振动．

如图，两个同心圆的半径分别为4cm和5cm，大圆的一条弦AB与小圆相切，则弦AB的长为（　　）