从E-R图导出时，如果两实体间的联系是M:N的，下列说法中正确的是
A．将M方关键字和联系的属性纳入N方的属性中
B．将N方关键字和联系的属性必定纳入M方的属性中
C．在M方属性和N方的属性中均增加一个表示级别的属性
D．增加一个关系表示联系，其中纳入M方和N方的关键字
参考答案：D
解析： 两实体间的联系是m:n时，关系模型是多对多联系。从E-R图导出时，需要把多对多联系分解成一对多联系，分解的方法就是增加一个关系表示联系，其中纳入m方和n方的关键字。
| 完形填空。 A group of frogs were traveling 1 the forests, but suddenly two of them fell into a hole. The other frogs tried to help them. When they saw how deep the hole was, they cried to the two frogs that they could not be saved. The two frogs didn't 2 and tried their best to jump up out of the hole. The other frogs 3 saying that they were sure to die. 4 , one of the two frogs, who heard what the other frogs were saying, gave up. Then he fell down and died. The 5 frog, however, continued to jump as 6 as he could, and at last made it out. When he 7 , the other frogs asked, "Didn't you hear us?" The frog, who had a poor 8 , said, "I thought you were encouraging me all the time." The story teaches us a 9 :There is power (力量) of life and death in the tongue. An encouraging 10 to those who are down can help them out while a discouraging word can kill them. | ||||
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