Theorem: If r and s are rational numbers, then the product of r and s is a rational number.

Which facts are assumed in a direct proof of the theorem?

r = a/b, and s = c/d, where a, b, c, d are integers and 0

rs = a/b, where a and b are integers

a=0.

and d = 0.

r = a/b, and s = c/d, where a, b, c, d are integers and a #0 and c #0

rs = a/b, where a and b are integers b ± 0.