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  3. If p and r are prime numbers, which of the following must also be prime?

If p and r are prime numbers, which of the following must also be prime?

admin2009-05-19  10
问题 If p and r are prime numbers, which of the following must also be prime?
选项 A、pr
B、p+r
C、pr+1
D、p2+r2
E、None of the above
答案E
解析 若p和r是质数(prime number)，则下面哪一个也一定是质数?
解：本题的正确答案为(E)。此题可用排除法。质数就是除了1和它本身之外，没有其他因子的数，因此(A)选项是显然不正确的；由5+7=12可以断定(B)也是错误的，实际上两个质数的和为偶数；5×7+1=36可认定(C)是错误的；(D)也肯定不是质数，所以也是错误的。
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