How to set Dropdownlist field in ASP.NET MVC

Question

i just learn about asp.net MVC. This web, is consume WCF Rest Services. Basically, this web is can Read and Write data from WCF Services. But when i go into edit form, all form is filled with model value except for dropdownlist.

First, let me show you my model:

public class Assignments
{
    [Display(Name="Subject")]
    public String asgn_subject { get; set; }

    [Display(Name = "Assignment Number")]
    public String asgn_id { get; set; }

    [Display(Name = "Trouble Ticket")]
    public String asgn_ticket { get; set; }

    [Display(Name = "Status")]
    public String asgn_status { get; set; }

    [Display(Name = "Start Date")]
    public String asgn_start { get; set; }

    [Display(Name = "Remark")]
    public String asgn_remark { get; set; }


    [DataType(DataType.DateTime)]
    [Display(Name = "End Date")]
    //[DisplayFormat(DataFormatString = "{0:dd/MM/yyyy hh:mm:ss tt}", ApplyFormatInEditMode = true)]
    public DateTime? asgn_end { get; set; }

    [Display(Name = "Acknowledge Time")]
    [DataType(DataType.DateTime)]
    //[DisplayFormat(DataFormatString = "{0:yyy-MM-dd hh:mm:ss}", ApplyFormatInEditMode = true)]
    public DateTime? asgn_ack_time { get; set; }
}

My ActionResult script:

public ActionResult Edit(string asgn_id)
        {
            AssignmentServiceClient asc = new AssignmentServiceClient();
            AssignmentsViewModels avm = new AssignmentsViewModels();

            avm.Assignments = asc.GetAsgn(asgn_id);

            return View("Edit", avm);  
        }

The model is same as the WCF Services model. The asgn_status is returning a string.

In the dropdownlist field, how to set the dropdownlist based on the model value ?

     @Html.DropDownListFor(model => model.Assignments.asgn_status, new List<SelectListItem>
               {new SelectListItem {Text="In Progress", Value="True", Selected=true},
               new SelectListItem{Text="Completed", Value="False"}}, new { @class = "form-control", style = "width:300px; height:35px;" })

EDIT

Get method

public Assignments GetAsgn(string asgn_id)
        {
            try
            {
                var webClient = new WebClient();
                string url = string.Format(BASE_URL + "GetAsgn/{0}", asgn_id);
                var json = webClient.DownloadString(url);
                var js = new JavaScriptSerializer();
                return js.Deserialize<Assignments>(json);

            }
            catch
            {
                return null;
            }
        }