Which method does following C# code explains?

[answerhappygod]

static void Main(string[] args) { int a = 10, b = 20; method(ref a, ref b); console.writeline(a + " " + b); } static void swap(ref int i, ref int j) { int t; t = i; i = j; j = t; }
a) Call by reference
b) Call by value
c) Output parameter
d) parameter arrays