When you implement two interfaces with the same method, how do you know which one is called?

Question

If you have TheMethod() in interfaces I1 and I2, and the following class

class TheClass : I1, I2
{
    void TheMethod()
}

If s

Answer 1

public interface IA
{
   void Sum();
}

public interface IB
{
    void Sum();
}

public class SumC : IA, IB
{
   void IA.Sum()
    {
        Console.WriteLine("IA");
    }

   void IB.Sum()
   {
       Console.WriteLine("IB");
   }
  public void Sum()
   {
       Console.WriteLine("Default");
   }
}

public class MainClass
{
    static void Main()
    {
        IA objIA = new SumC();
        IB objIB = new SumC();
        SumC objC = new SumC();

        objIA.Sum();
        objIB.Sum();
        objC.Sum();

        Console.ReadLine();
    }
}