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In my opinion, there's a flaw in the Official Explanation. We all know that 0 is divisible by any number, except 0. My answer is A. Here is why.
(1) n2 + n = n(n+1) is not divisible by 3
==> n = 3m + 1 & n+1 = 3m + 2, where m is an integer.
==> n-1 = 3m is divisible by 3
==> Sufficient
(2) 3n+5 >= k + 8, where k is a positive multiple of 3
==> n >= k/3 + 1
Given that k is a positive multiple of 3, we know that n must be greater than or equal to 2. We get no further information about n.
==> Insufficient
From (1) & (2), answer is A.
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