方法都报错 报错原因(找不到合适方法重写)

using System;

namespace 基类和派生类_最高派生版本_
{
    class Program
    {
        static void Main(string[] args)
        {
            DerivedClassTwo op = new DerivedClassTwo();
            BaseClass mc = (BaseClass)op;
            op.d();
        }
    }
    class BaseClass
    {
       virtual public void a()
        {
            Console.WriteLine("我是基类a方法");
        }
    }
    class DerivedClass :BaseClass
        {
           override public void c()
            {
                Console.WriteLine("我是基于第一个基类派生类C方法");
            }
        }
        class DerivedClassTwo : DerivedClass
    {
        override public void d() { Console.WriteLine("我是基于第二基类派生类D方法"); }
        }

}