Da

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            B b = new B();

            string firstResult = b.Result();
            string secondResult = ((A)b).Result();

            Console.WriteLine(firstResult);
            Console.WriteLine(secondResult);
        }
    }

    class A
    {
        public string Result()
        {
            return "A";
        }
    }

    class B : A
    {
        public string Result()
        {
            return "B";
        }
    }
}