难道又是我的智商问题？！

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)     2 is
not a factor of n.

(2)     3 is
not a factor of n.

答案是C

满足1,则：n为奇数，n-1，n+1中一个仅能被2整除，一个能被4整除。

满足2，则：n+1，n-1必有一个能被3整除。

同时满足1，2则r=0