X.25协议定义了（5）层结构，它所采用了的流控和差错控制协议是

The first two stages in the development of civilized man were probably the invention of primitive weapons and the discovery of fire, although nobody knows exactly when he acquired the use of the latter.
41) ______ Animals have a few cries that serve as signals, but even the highest apes have not been found able to pronounce words, even with the most intensive professional instruction. The superior brain of man is apparently a necessity for the mastering of speech. When man became sufficiently intelligent, we must suppose that he gradually increased the number of cries for different purposes. It was a great day when he discovered that speech could be used for narrative. There are those who think that in this respect picture language preceded oral language. A man could draw a picture on the wall of his cave to show in which direction he had gone, or what prey he hoped to catch. 42) ______
Two important stages came not so long before the dawn of written history. The first was the domestication of animals; the second was agriculture. Agriculture made possible an immense increase in the number of the human species in the regions where it could be successfully practiced. 43) ______
44) ______
These inventions and discoveries--fire, speech, weapons domestic animals, agriculture, and writing--made the existence of civilized communities possible. From about 3000 B. C. until the beginning of the Industrial Revolution less than two hundred years ago there was no technical advance comparable to these. During this long period man had time to become accustomed to his technique, and to develop the beliefs and political organizations appropriate to it. There was, of course, an immense extension in the area of civilized life. At first it had been confined to the Nile, the Euphrates, the Tigris, and the Indus, but at the end of the period in question it covered much the greatest part of the inhabitable globe. I do not mean to suggest that there was no technical progress during the time. 45) ______
[A] Probably picture language and oral language developed side by side. I am inclined to think that language has been the most important single factor in the development of man.
[B] Another fundamental technical advance was writing, which, like spoken language, developed out of pictures, but as soon as it had reached a certain stage, it was possible to keep records and transmit information to people who were not present when the information was given.
[C] With the development of civilization, primitive people who lived in caves at that time badly needed a language, which would help them to communicate with one another.
[D] The origin of language is also obscure. No doubt it began very gradually.
[E] In fact, there was progress--there were even two inventions of very great importance, namely, gunpowder and the mariner’s compass—but neither of these can be compared in their revolutionary power to such things as speech and writing and agriculture.
[F] These were, at first, only those in which nature fertilized the soil after each harvest. Agriculture met with violent resistance from the pastoral nomads, but the agricultural way of life prevailed in the end because of the physical comforts it provided.
[G] But industry was a step in human progress to which subsequently there was nothing comparable until our own machine age.

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