求助 CSS 问题

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protected void CreateImage()
    {
        for (int m = 0; m < 3; m++)
        {
            ImageButton bt = new ImageButton();
            bt.CssClass = "img";
            bt.Width = 71;
            bt.Height = 96;
            bt.ImageUrl = "i/" + m.ToString() + ".jpg";
            bt.Click += new ImageClickEventHandler(bt_Click);
            Panel1.Controls.Add(bt);
        }
    }
    void bt_Click(object sender, ImageClickEventArgs e)
    {
        ImageButton bt = (ImageButton)sender;
        string ss = bt.ImageUrl;
        if (bt.CssClass == "imagebutton")
            bt.CssClass = "img";
        else
            bt.CssClass = "imagebutton";

    }
按钮数为1时运行正常，多个时无效。