Bunuel wrote:

k is a positive integer. Is k prime?

Notice that we are told that k is a positive integer.

(1) At least one number in the set {1, k, k + 7} is prime. Clearly insufficient.

(2) k is odd. Clearly insufficient.

(1)+(2) The set is {1, k=odd, k+7=even>2}. Since at least on number in this set is a prime, then it must be k. Sufficient.

Answer: C.

I thought it was A because I thought that in a set, the numbers are organized by size. So I assumed that k is prime because any prime + 7 = no prime. But obviously a set is not sorted. I get the rest of the reasoning. Am I right assuming that sets don't need to be sorted? Please verify!

Thanks!