protected void Button3_Click(object sender, EventArgs e)
    {
        test(sender);
    }
    protected void Button4_Click(object sender, EventArgs e)
    {
        test(sender);
    }
    protected void Button5_Click(object sender, EventArgs e)
    {
        test(sender);
    }

    private void test(object sender)
    {
        //这里怎么根据sender判断是点击button1进来的？
        Button btn = (Button)sender;
        Response.Write(btn.ID.ToString() + "被点击了");
    }