问题
填空题
关系代数是一种关系操纵语言，它以集合代数为基础发展起来，但它的操作对象和操作结果均为 【10】 。
答案
参考答案：集合
解析： 关系操作的特点是集合操作关系，即操作的对象和结果都是集合。
搜题请搜索小程序“爱下问搜题”
关系代数是一种关系操纵语言，它以集合代数为基础发展起来，但它的操作对象和操作结果均为 【10】 。
参考答案：集合
解析： 关系操作的特点是集合操作关系，即操作的对象和结果都是集合。
| 完形填空。先通读下面的短文，掌握其大意，然后在每小题所给的A、B、C、D四个选项中，选出可以填入空白处的最佳选项。 | ||||
| Thousands of years ago, people did not know 1 to make a fire. When it was very 2 at night, they could not sleep well. Sometimes lightning hit (打，击) a 3 and started a fire. At first people were both surprised and 4 to see the fire. They were afraid to go up. Later a few 5 men took some of this fire and made a fire near their homes. They used the fire to keep them warm and to cook food. When they had a fire, they did not let it 6 burning. If it stopped, they could not start it again. Maybe they had to 7 for a long time, sometimes for 8 . Later they 9 how to make a fire with pieces of wood. Now we have matches, we can put them in our pockets and make a fire when we want to make it. But we have to be 10 with fire. | ||||
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