Why are signatures declared in the base class ignored?

class Base
{
    public virtual void MethodA(int x)
    {
        Console.WriteLine ("In Base Class");
    }
}

class Derived : Base
{
    public override void MethodA(int x)
    {
        Console.WriteLine ("In derived INT)");
    }

    public void MethodA(object o)
    {
        Console.WriteLine ("In derived OBJECT");
    }
}

class Test
{
    static void Main()
    {
        Derived d = new Derived();
        int k = 20;
        d.MethodA(k);
    }
}

The output I got for this is "In derived OBJECT". What is the reason for this strange behaviour? After some research, I found out the reason is the signatures declared in the base class are ignored. Why are they ignored?