Question 4 (14 points) Let a,b be 2 integers not both 0. In the proof that the GCD of a and b is a linear combination of a and b. we start by defining S - {ra + sbr, s c Z, ra + sb - 0. We show that S in not empty so that S has a smallest element. which we call d. In the next step.we show that d divides a. Provide the details of this step