题目内容
在一个基本表上最多只能建立一个聚簇索引。
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连接条件中的各连接字段类型必须是相同的。
（一）If we were asked exactly what we were doing a year ago, we should probably have to say that we could not remember. But if we had kept a book and had written in it an account of what we did each day, we should be able to give an answer to the question. It is the same in history. Many things have been forgotten because we do not have any written account of them. Sometimes men did keep a record of the most important happenings in their country, but often it was destroyed by fire or in a war. Sometimes there was never any written record at all because the people then did not know how to write. For example, we know a good deal about the people who lived in China 4, 000 years ago, because they could write and leave written records for those who lived after them. But we know almost nothing about the people who lived even 200 years ago in central Africa, because they had not learned to write. Sometimes, of course, even if the people cannot write, they may know something of the past. They have heard about it from older people,and often songs and dances and stories have been made about the most important happenings. And these have been sung and acted and told for many generations for most people are proud to tell what their fathers did in the past. This we may call ‘remembered history’. Some of it has now been written down. It is not so exact or so valuable to us as written history is, because words are much more easily changed when used again and again in speech than when copied in writing. But where there are no written records, such spoken stories are often very helpful. 1. Which of the following ideas is not suggested in the passage?.
We know very little about the central Africa 200 years ago because.
“Remembered history” is considered valuable only when .
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