The Internet has become part of teenage life. There’s a report on 3,375 students aged from 10 to 18 in seven Chinese cities. It says that 38% of them believe they use the Internet often. While most of them get useful information and use the Internet to help in their studies, some are not using it in a good way. Many are playing online games too much. A few even visit Web sites they should not look at. Bad things can happen if young people spend too much time on the Internet.
   In order to help young people use the Internet in a good way, a textbook on good Internet behavior has started to be used in some Shanghai middle schools this term. It uses real examples to teach students all about good ways of using the Internet. Some students also make online friends. But if you are meeting a friend offline, let your parents know.
   Teachers and parents all think the book is a very good idea. It will teach students how to be a good person in the online world. It will be a guide for teens using the Internet and keep students away from bad sites.

一杯热水的降温规律是____。

在$∆ABC$中，$a=3,b=\sqrt{7},c=2$，那么B等于(      )

用墨汁将细菌染色后放入草履虫培养液中，用显微镜观察草履虫，发现黑色结构是（    ）