Understanding interface in C#

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C#

.NET

Hello Folks,
Today I was going through concepts of Interfaces in C# and came across an example on Interface. Please see the following:

Interface code:

using System;
using System.Collections.Generic;
using System.Text;
namespace ClassLibrary1
{
    public interface IAClass
    {
        int AddNumber(int a, int b);
    }
    public interface IBClass
    {
        int AddNumber(int a, int b);
    }
    public class Class1: IAClass, IBClass
    {
        public int AddNumber(int a, int b)
        {
            return a + b;
        }
    }
}

Console Application Code:

using System;
using System.Collections.Generic;
using System.Text;
using ClassLibrary1;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            IBClass obj = new Class1();

            Console.WriteLine(obj.AddNumber(3, 6));
        }
    }
}

I am getting the output as 9, which is correct. Now my question is that I have declared same method in both interfaces, with same signature. Class1 is implementing both Interfaces. If I am creating the object of IAClass or IBClass output is same, how does the compiler understand that which method it should call?
Why is it not throwing any error?