If two the four expressions x+y,x+5y,x-y, and 5x-y are chosen at random, what is the probability that their product will be of the form of
sqr(x) - sqr(by), where b is an integer?

a. 1/2
b. 1/3
c. 1/4
d. 1/5
e. 1/6

answer is 1/6.