using static System.Console;

namespace ConsoleApplication2
{
    public interface IActivity
    {
        void DoActivity();
    }

    public class A : IActivity
    {
        public virtual void DoActivity()
        {
            WriteLine("A does activity");
        }
    }

    public class B : A, IActivity
    {
        public override void DoActivity()
        {
            WriteLine("B does activity");
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            IActivity ia = new B();
            ia.DoActivity();
            ReadKey();
        }
    }
}