how return Model with html.beginForm in mvc4

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C#

ASP.NET

MVC

hi
i write this code in HttpGet Index Control :

[HttpGet]
        public ActionResult Index(int id)
        {
		var model=_db.nezam.tolist();
		return view(model);
		}

and in view i have a form :

@using(Html.BeginForm()){
<input type="text" name="cap" />
<input type="submit"  value="OK" />
}

in httpost i write :

[HttpPost]
    public ActionResult Index(string cap)
      {
          string st=HttpContext.Session["captcha"].ToString();
          if (cap == st)
          {
              return Content("true");
          }
          return View(?);
      }

when i run my project and click on ok button give me this error :

System.NullReferenceException: Object reference not set to an instance of an object.

it is true because i dont sent any model to view , but i dont know how get model from html.beginform and next send it to view again ,=== return View(?)?????