Hal has 4 girl friends and 5 boy friends. In how many different ways can Hal invite 2 girls and 2 boys to his birthday party?

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问题 Hal has 4 girl friends and 5 boy friends. In how many different ways can Hal invite 2 girls and 2 boys to his birthday party?

选项 A、54
B、60
C、72
D、120
E、240

答案B

解析第一步选择女孩，从4个人里面选择24个人C₄²；第二步选择男孩，从5个人里面选择2个人C₅²。
根据乘法原理可以得出C₄²×C₅²=60。所以答案为B选项。

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