C# 抽象类继承问题

Question

很小的一段代码。。但我太菜了，看不出问题using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;namespace Animal
{
    class Program
    {
        public interface Animal
        {
            void Move();
        }         abstract class Cat: Animal 
        { 
            public void Eat(){Console.Write("Eat!");}
        } 
          public class Use : Cat
           {
               public override void Move()
               {
                   throw new NotImplementedException();
               }
           }
       
        static void Main(string[] args)
        {
            Use u = Use();
            u.Move();
        }
    }
}
编译之后，报错：Cat类比Use类可访问性低。  什么意思啊？？

Answer 1

abstract class Cat: Animal 
        { 
            public void Eat(){Console.Write("Eat!");}
        } =>
public abstract class Cat: Animal 
        { 
            public void Eat(){Console.Write("Eat!");}
        } 

Answer 2

        public abstract class Cat: Animal 
        { 
            public void Eat(){Console.Write("Eat!");}
        } 

Answer 3

 public interface Animal //public
        {
            void Move();
        }        public abstract class Cat: Animal //不加public,默认是private
        { 
            public void Eat(){Console.Write("Eat!");}
        } 
          public class Use : Cat  //public
           {
               public override void Move()
               {
                   throw new NotImplementedException();
               }
           }//cat是private,而use是public,访问级别当然不一样

Answer 4

类默认访问级别是protected
子类访问性要比父类高