This was probably one of the easiest GMAT questions I have done and has renewed my faith in getting a decent score.
1. X-Y A is clearly insufficient because the numbers could be all over the place. 11-9, 7-5, 5-3
2.y>x>6 (2 sets of numbers fulfill this scenario 5+3 no prime and 5+ 2 = prime) since 1 cannot be prime. B is insufficient
Thus D is insufficient
C or E
5-3 = prime 5-2 = prime
Since you have two possibilities C is insufficient
Thus E is the only answer.
1. X-Y A is clearly insufficient because the numbers could be all over the place. 11-9, 7-5, 5-3
2.y>x>6 (2 sets of numbers fulfill this scenario 5+3 no prime and 5+ 2 = prime) since 1 cannot be prime. B is insufficient
Thus D is insufficient
C or E
5-3 = prime 5-2 = prime
Since you have two possibilities C is insufficient
Thus E is the only answer.