What is the output of the following:

public class A
{
    public int num { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        A a = new A();
        a.num = 12;
        ChangeString(a);
        Console.WriteLine(a.num);
    }

    public static void ChangeString(A obj)
    {
        obj.num = 99;
    }
}