[求助]请教几道JJ数学题

1.            If n is an integer greater than 6, which of the following must be divisible by 3?

a.            n (n+1) (n-4)

b.            n (n+2) (n-1)

c.             n (n+3) (n-5)

d.            n (n+4) (n-2)

e.            n (n+5) (n-6)

2.            n是正整数，
                        问以下哪个是3的倍数?

a.            (n-1)n(n+1)(n-3)

b.            n(n2-1)

c.             3n-n

答案是 1. A    2. A, B   但是我自己没有什么头绪, 希望NN能指导一下这种题的思路. 谢谢