if x and y are two different integers in S, which of the following must be the square of an integer
I. (x-1)(y-1)
II. 4(x-1)
III. x^2+y^2
a. none
b. I only
c. II only
d. III only
e. I and II
try x=4, y=5
a. 12 not square
b. 12 not square
c. 16+25=41 not square.
answer a.