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Mobile phones help us communicate everywhere, which is very important. They are good at emergencies, when you’ve (21) a car accident, for instance. A mobile phone (22) save your life! Mobile phones are also (23) because they have many good features--we can take photos and send them all over the world. But every coin has two sides. The (24) thing about mobile phones is that you have no privacy. My boss and friends find me whenever they want! I haven’t enjoyed a whole quiet day since the day I (25) my mobile phone. (26) disadvantage is that mobile phones make us lose good habits, such as punctuality. Before, when we (27) keep in touch so easily, if we had a date with someone, we tried our best to arrive (28) time. But now, because of the mobile phone, we are not afraid of (29) late any more, so, surprisingly enough, we make people wait! The third (30) I see is that I change my mobile phone once a year, and this means I’m spending (31) too much money on it. I think it is important to think (32) we use mobile phones. We should use things such as mobile phones, laptops, cameras, the Internet, and so on, (33) our needs. We can (34) our mobile phone when we don’t want to be disturbed by (35) . If we know how to use it, we can eliminate its disadvantages. 21（）
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Online distance learning is an instructional system which connects learners with educational resources. Students work on their own at home, at work, or at school and communicate with faculty and other students by means of e-mail, electronic meetings, videoconferencing, chat rooms, instant messaging and other forms of computer-based communication. There are both advantages and disadvantages to online distance learning.There are many benefits to using online distance learning environments. Online education is available all the time, anywhere and to all people. However, there are weaknesses for some learners. The online learner only has the written text and no other face-to-face hints. This may confuse the learner and cause misunderstanding. While distance learning allows for an openness, it is also difficult because it is done by e-mail messages and writing, and therefore may take more time than face-to-face learning. Sometimes the messages can be overwhelming for many online students.Universities, colleges and schools use online distance learning environments. These are important for students who may be unable to attend classes for various reasons like illnesses or busy everyday schedules. Some learners just want to further their studies at home. They enjoy the convenience of home learning as they take regular programs or enrichment classes.Online courses keep learners very occupied at all hours of the day. There are a great many messages and other online resources to read and respond to. Most learners have regular jobs or attend regular school classes on ground as well. However, the benefits are clear. Online distance learning is becoming very popular. Some online classes have become a profitable business as they replace regular traditional means of learning. One of the good aspects of online distance learning is（）
Online distance learning is an instructional system which connects learners with educational resources. Students work on their own at home, at work, or at school and communicate with faculty and other students by means of e-mail, electronic meetings, videoconferencing, chat rooms, instant messaging and other forms of computer-based communication. There are both advantages and disadvantages to online distance learning.There are many benefits to using online distance learning environments. Online education is available all the time, anywhere and to all people. However, there are weaknesses for some learners. The online learner only has the written text and no other face-to-face hints. This may confuse the learner and cause misunderstanding. While distance learning allows for an openness, it is also difficult because it is done by e-mail messages and writing, and therefore may take more time than face-to-face learning. Sometimes the messages can be overwhelming for many online students.Universities, colleges and schools use online distance learning environments. These are important for students who may be unable to attend classes for various reasons like illnesses or busy everyday schedules. Some learners just want to further their studies at home. They enjoy the convenience of home learning as they take regular programs or enrichment classes.Online courses keep learners very occupied at all hours of the day. There are a great many messages and other online resources to read and respond to. Most learners have regular jobs or attend regular school classes on ground as well. However, the benefits are clear. Online distance learning is becoming very popular. Some online classes have become a profitable business as they replace regular traditional means of learning. Sometimes online distance learning can be a problem because （）
在加密技术中，作为算法输入的原始信息称为______。
认证的主要目的有两个：第一是信源识别：第二是______。
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