谁帮我解决下这段代码中的问题？

代码如下：

public class Animal
{
    public string sflag;
    public virtual void Bit(){sflag = "This is an animal.";}
}
public class Cat:Animal
{
    public override void Bit(){sflag = "This is a cat.";}
}
public class Dog:Animal
{
  public override void Bit(){sflag = "This is a dog.";}
}

问题:
1.为什么Animal中要用public virtual void Bit(){sflag = "This is an Animal.";}而Cat和Dog中要用public override void Bit(){sflag = "This is a cat.";}和public override void Bit(){sflag = "This is a Dog.";}

2.public override void Bit()和public override void Bit()有什么区别?

3.为什么有些public后有void而有些又没有?有void和没有void有什么区别?

就这三个问题,麻烦高手帮忙解决一下!谢谢!