如图，AB为⊙O的弦，⊙O的半径为5，OC⊥AB于点D，交⊙O于点C，且CD=1，则弦AB的长是  ____．

小张在日记里写道：“放学时，铃声是悦耳的；考试时，铃声是紧张的；思考时，铃声是刺耳的。”这说明（    ）

根据课文《桂林山水》内容填空。

与“桂林山水甲天下”相呼应的句子是：____。

    Dan and Kim are friends. Dan lives in an apartment downtown(市中心). It is very convenient to go to work or go shopping, but it's too noisy. There is a lot of traffic. Sometimes Dan can't stand(忍受). Kim has a house in the suburbs(郊区). It's not convenient to go to work or go shopping, but it's very quiet. There is not much to do. In fact, nothing ever happens.

    Dan and Kim have a good idea. They decide to trade(决定交换)places one week.