有3户人家共订了10份日报，每户人家至少2份，最多4份。问：一共有多少种不同的订法______
A．6
B．12
C．18
D．21
参考答案：A
解析：该题属于排列组合问题。已知3户人家共订了10份日报，且要求每户人家至少2份，最多4份。因此，“至少2份，最多4份”的数量只能是2，4，4或3，3，4。而第一种情况3户人家又有3种不同的分法，即2，4，4；4，2，4；4，4，2。第二种情况同样也有3种不同的分法，即3，3，4；3，4，3；4，3，3。因此，总共有：3+3=6(种)不同的订法。故选A。
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| The Sound of Music is one of the most popular American movies. It is the story 1 Maria. She was a young woman. She went to 2 a family with seven children. The 3 name was Von Trapp. They lived in 4 . Mrs. Von Trapp died, and the family 5 tired. The children were often unhappy. Their crying and shouting often make their father 6 . Maria was able to 7 the family by teaching them to sing 8 songs and performing short, crazy plays. The father was sad 9 , 10 the smiling faces of his children pleased him and made him happy again. | ||||
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