如图，在Rt△ABC中，∠ACB=90°，CD是AB边上的高，AC=6，AB=9，则AD=（　　）

阅读理解
   The best thing about going to school is learning how to read. Books are great friends. They can help you to learn about anything you want to know. They can go anywhere with you. You can read about animals, faraway places, people, and anything else.
   Books are written by people called authors(作者). Authors think of things they want to write about. They make a plan for a book and start to write .Some authors write books that give us information(信息)and teach us things. Authors are very important people because they help us learn. Find a good book to read and read every day ,You will enjoy your time with a good book.

下列对文章《巴尔扎克作品、〈水浒〉、〈红楼梦〉的对话艺术》的理解和分析，错误的两项是(　　)。