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现有一种密码,它是由3个a,2个b,1个c和1个d组成的七位代码,则这种密码的个...
现有一种密码,它是由3个a,2个b,1个c和1个d组成的七位代码,则这种密码的个数是( )
A.120
B.240
C.360
D.420
考点分析:
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