Given the following code to compute "n to the power of x"

 
static int fastPower(int n, int x){
		int a= x; int b= 1; int i= n;
		while(i>0) {
			if (i%2==0){
				a= a*a;
				i= i/2;
			}
			else {
				b= a*b;
				i= i-1;
			}
		}
		return b;
	}

Which of the following logical statements is a valid and useful invariant for proving this code does actually compute n to the power of x?