请问下怎么清除cookie？

交谈中请勿轻信汇款、中奖信息、陌生电话，勿使用外挂软件。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace cookie1
{
public partial class WebForm1 : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{

}

protected void Button1_Click(object sender, EventArgs e)
{
HttpCookie cook = new HttpCookie("usename");
cook.Value = TextBox1.Text;
cook.Expires = DateTime.Now.AddDays(30);
Response.Cookies.Add(cook);
}

protected void Button2_Click(object sender, EventArgs e)
{

HttpCookie cooks = Request.Cookies["usename"];
if ( cooks== null)
{
TextBox2.Text = "无值！";
}
else
{
TextBox2.Text =cooks.Value;
}
}

protected void Button3_Click(object sender, EventArgs e)
{
Request.Cookies["usename"].Expires = DateTime.Now.AddDays(-1);
// Request.Cookies.Remove("usename");//清除指定Cookie
//Request.Cookies.Clear();//清除所有Cookie
}

}
}


button3里的三个方法都试过了，不行。谢谢大侠，小弟非常菜鸟。