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在对局域网的运行与维护前，首先要判断其拓扑结构和信道访问方式，然后要判断其故障类型，网络故障的分类方式不尽相同，一般可以根据故障的性质分为 (56) 和 (57) 。随着Internet的广泛应用，为了保证网络稳定运行，网络安全性尤为重要，而防火墙是一种综合性的技术，涉及计算机网络技术、密码技术、安全技术、软件技术、安全协议、网络标准化组织的安全规范以及安全操作系统等多方面，防火墙技术一般可以分为两类，即 (58) (采用报文动态分组)和 (59) (采用代理服务机制)，而后者又包括 (60) 。

56（）

答案

参考答案：物理故障

单项选择题

How to protect children Web fans from unsuitable material on-line while encouraging them to use the Internet has long been discussed in the US.
For some parents, the Internet can seem like a jungle, filled with danger for their children. But jungles contain wonders as well as dangers and with good guides, some education, and a few precautions, the wilds of the Internet can be safely navigated. "Kids have to be on-line. If we tell our kids they can’t have access to the Internet, we’re cutting them off from their future, " said an expert.
Most kids have started to use search engines. Many of them are great for finding tons of interesting Internet sites, and they can also locate places where you might not want your kids to go. There are search engines designed just for kids. A certain software contains only sites that have been selected as safe. The most popular way to limit access would be to use what is known as a "content screener". But this can’t be wholly reliable, and the best thing parents can do is to talk to their kids and let them know what is OK or not OK to see or do on the Internet. Another way is that mum or dad is nearby when the child is surfing the Internet.
A few other tips
·Don’t put the PC in a child’s room but keep it in an area where mum or dad can keep an eye on things. That also makes the Internet more of a family activity.
·Ask your child what he or she has been doing and about any friends they make on-line.
·Tell your child not to give on-line strangers personal information, especially like address and phone number.
·And tell your children never to talk to anyone they meet on-line over the phone, send them anything, accept anything from them or agree to meet with them unless you go along.

The best way to protect children from improper material is ______.

A. to install (安装) a content screener on the computer
B. to buy some search engines for the children
C. to be nearby when they are surfing the Internet
D. to talk to the children and persuade them to tell right from wrong
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问答题

我们都有这样的生活经验，食品放置一段时间后就会腐败变质。这是细菌和真菌在食品中大量繁殖的结果。怎样才能在不影响食品的营养价值和风味的情况下，消灭或抑制其中的微生物，从而延长食品的保存期呢？目前常用的保存食品的方法有罐藏法、 巴氏消毒法、冷藏法、干燥法等。

试说出上述各种方法所依据的原理，并举例说明。

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