跳楠是蝗虫的幼虫，它与成虫的区别是（    ）

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    This is Hong Kong Middle School. It is very big and beautiful. There are 34 classes, 300 teachers and 3, 500 students in the school. In the middle of the school there is a tall building. It's for us to have classes. All the classrooms are there. There are some small rooms, too. They are offices for the teachers to work. All the teachers get ready for their lessons there.

    In front of the tall building, there is a small one. It is the library. There are many books there. The teachers and the students like to read books in it. Sometimes they borrow(借)books from it.

    There is a dining hall behind the tall building. Some of the students have lunch in it.

    I like my new school very much.

读珠江三角洲、港澳地区图及两地分工合作模式图，回答问题。

如图所示，已知圆O的直径AB=$\sqrt{6}$，C为圆O上一点，且BC=$\sqrt{2}$，过点B的切线交AC延长线于点D，则DB=____