probability - gmat prep

[g.shankaran]

if two of the four expression x+y, x+5y , x-y and x-5y are chosen at random, what is the probability that their product will be of the form of x^2 - (by)^2 , where b is integer?

1. 1/2
2. 1/3
3. 1/4
4. 1/5
5. 1/6

[cans]

(x+y)(x-y) and (x+5y)(x-5y) are 2 such pairs
total=4C2=6
thus prob=2/6=1/3
IMO B