急问一道GMATPREP上的数学题？

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If n is a positive integer and r is the remainder when (n-1)(n+1) is divided by 24, what is the value of r?

1. n is not divisible by 2.

2. n is not divisible by 3.

Answer: c

Why? 请好心人帮忙！谢谢先！

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不好意思，已在置顶的“GMATPREP专题”中找到解答，感谢"CD小兵"的辛勤工作，让我们受益多多。