下图示意浙江省某地区等坡度线（地表坡度值相等的点连成的线），图中数字代表坡度（坡面与水平面的夹角）。图中河流，流速最快的河段是(     )

我们传递信息的方式多种多样，像最普遍的有____、____、标牌、____以及哑语手势等。

观察下列各式：

$\sqrt{1+\frac{1}{1^2}+\frac{1}{2^2}}$  =1+ $\frac{1}{1}$ ﹣ $\frac{1}{2}$  =1 $\frac{1}{2}$

$\sqrt{1+\frac{1}{2^2}+\frac{1}{3^2}}$  =1+ $\frac{1}{2}$ ﹣ $\frac{1}{3}$  =1 $\frac{1}{6}$

$\sqrt{1+\frac{1}{3^2}+\frac{1}{4^2}}$  =1+ $\frac{1}{3}$ ﹣ $\frac{1}{4}$  =1 $\frac{1}{12}$

请你根据上面三个等式提供的信息，猜想：

根据短文内容，选择正确最佳答案。
American Classroom Rules
1. If the teacher asks a question, you should give an answer. If you do not understand the question, you should put up your hand and ask the teacher to repeat the question. If you don't know the answer, it's all right to tell the teacher that you don't know. Then he or she knows what you need to learn.
2. There is no excuse for not doing your homework. If you can not go to school, you should call your teacher or your classmate and ask for the homework. It is your responsibility（责任）to find out what homework you have missed. It is not the teacher's job to remind you of missed homework.
3. You must not be absent(缺席) on a test day. If you are badly ill, call and let the teacher know you will not be there for the test. If your teacher allows you to take a makeup, you should take the test within one or two days after you return to class. Bad illness is the only reason for missing a test.
4. Be on time! It's not polite to be late. Also, it bothers other students. If you must come in late, be sure to do it quietly. Have your books out of your bag before you come into the classroom. Then go to your seat and sit quietly. In the US, it is not necessary to knock at the door before you come into the classroom.