a²-1=4n,因为 a²-1=（a-1)(a+1),条件给出a是奇数,所以a-1和a+1均为偶数,即 a²-1为4的倍数.又因为(a+1)－（a-1)=2,所以(a+1)／2－（a-1)／2＝1,所以(a+1)／2与（a-1)／2为两个相邻的整数,其奇偶性相反,所以[(a+1)／2]×[（a-1)／2]为2的倍数,不是4的倍数.
即﹙ a²-1﹚／4为2的倍数且不为4的倍数,即a²-1为8的倍数不为16的倍数.所以选C.
If a is an odd number,then there must exist an integer such
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