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答案：
C
题目问的是
：Q语言的code words 是不是有且只有3个不同的符合组成？
条件（1）：Q语言的所有code words的集合是所有不同的由1个或多个符号构成的水平排列的集合。那么，可以理解为：Q语言的code words由1个或者多个符号水平排列而成。
条件（2）：Q语言中有且只有15个code words
条件(1)和条件(2)单独均不能确定是否只有3个不同符号，但是如果两条件together，分析和计算过程如下：
1个符号时的排法数：1
2个符号时的排法数：A(2,1)+A(2,2)=2+2=4
3个符号时的排法数：A(3,1)+A(3,2)+A(3,3)=3+6+6=15
……
n个符合时的排法数：A(n,1)+A(n,2)+...+A(n,n)
综上所述，n为其他值的时候均不能满足。
因此两条件together is are sufficient，选C。 
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