This data sufficiency question has been on my mind for a week now, can anyone please help? ( it came from the official GMAT quantitative book). I've reviewed the provided answer, but still dont' understand the explanation. Does GMAT assume that all sets described in its questions are evenly spaced?

K is a set of numbers such that

i) if x is in k, then -x is in k, and
ii) if each of x and y is in K, then xy is in K

is 2 in K?

1) 2 is in K
2) 3 is in K