The type or namespace name 'EventArgs' could not be found

i'm getting this error [The type or namespace name 'EventArgs' could not be found (are you missing a using directive or an assembly reference?)]
//using System;//using System.Data;
//using System.Web;
//using System.Web.Security;
//using System.Web.UI;
//using System.Web.UI.Page;
//using System.web.Security;
//using System.Collections;
//using System.Configuration;
//using System.Web.UI.HtmlControls;
//using System.Web.UI.WebControls;
//using System.Web.UI.WebControls.WebParts;



public partial class admin_login : System.Web.UI.Page{    protected void Page_Load(object sender, EventArgs e)
{   
if (IsPostBack)   
{     
  Label1.Visible = true; 
  }
}   
protected void LoginButton_Click(object sender, EventArgs e)   < ...