What is the following function determining?

1.10 What is the following function determining?
int fn(int a, int b) {
if (b = = 0) return 0;
if (b = = 1) return a;
return a + fn(a, b-1);
}
A) a + b where a and b are integers
B) a + b where a and b are non-negative integers
C) a * b where a and b are integers
D) a * b where a and b are non-negative integers