公开密钥方法的主要优点之一是(46)。RSA算法的基础是(47)。当N个用户采用公开密钥方法保密通信时，系统中一共有(48)个密钥，每个用户要小心保管好(49)个密钥。为了防止不者实用户否认他们曾经通过计算机发送过文件，较简便的方法是利用公开密钥方法完成(50)。

听力原文：M: Hi, Grace. Mind if I cat lunch with you?W: No. Mr. Evans, not at all.M: Thanks. I just beard that you're studying nutrition and you've got quite a bit of experiences working in the cafeteria, so I wonder if you will be interested in a small project we are doing this germ.W: What's the project all about?M: More and more students have been deciding not to buy the meal here and we want to attract them back. So I want to hear what students would like. Your job would be to find out.W: Well, if the menus were changed, then maybe I wouldn't have to listen to so much criticism.M: That makes you perfect for the job. Would you be interested?W: I'm not sure. What sorts of changes are you thinking of?M: I'd like to make some changes in the way we prepare our food. For example, just look at what we have to choose from today. You got a fried hamburger and I got fried chicken; They both contain too much fat.W: But you'd better not get rid of them. They're everybody's favorite.M: Well, we can certainly keep them, but we need to give the people who are health-conscious some choices. For example, we could also prepare chicken without the fatty skin and serve it on some rice with a light sauce. Do you think that would appeal to students?W: Well, I'd like that. You're right. You'd better find out what others think. Sorry, I've got to get hack to work. I'd like to hear more though. I'll drop by your office later.M: OK. See you then.(23)A．The size of the cafeteria.B．The food served in the cafeteria.C．The cost of meals in the cafeteria.D．Career opportunities in cafeterias. 点击查看答案

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