In the following example: 

```
class Base
{
    public void TestMethod()
    {
        Console.WriteLine("Base Class");
    }
}

class Derived : Base
{
    public void DerivedTestMethod()
    {
        Console.WriteLine("Derived Class");
    }
}

class Demo 
{
    public static void Main()
    {
        Base Obj = new Derived();
        Obj.DerivedTestMethod(); //Error Line
    }
}

```

Why can't I access the method DerivedTestMethod() in Derived class when I have created the object for Derived class using **new**?