面向对象多态是怎么一回事？

public class A
{
  public virtual void Method ()
  {
    //打印字符串A
  }
}public class B : A
{
  public override void Method ()
  {
    //打印字符串B
  }
}public class Program
{
  public static void Main(string[] args)
  {
    A a = new B();
    a.Method();//打印字符串B
  }
}