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? a, r = a/b, and s-c/d, where a, b, c, d are integers and a # 0 and c#0. brs-a/b, where a and b are integers a 0. c. r = a/b, and s-c/d, where a, b, c, d are integers and b 0 and d 0. drs-a/b, where a and b are integers b 0