Sharepoint webpart dropdownlist selectedindex value always -1 on buttonclick

$(document).ready(function () {
       FillDropDown();
   });
 function FillDropDown() {
       var index = 0;
       $("#filterCountry").empty();
 $().SPServices({
           operation: "GetList",
           listName: "Pages",
           async: false,
           completefunc: function (xData, Status) {
               $(xData.responseXML).find("Field[DisplayName='Country'] CHOICE").each(function () {
                   //if (index == 0) {
                   //    $(".filterCountry").append("<option value='0'>--Select--</option>");
                   //}
                   listItem = "<option value=" + $(this).text() + ">" + $(this).text() + "</option>";
                   $("#filterCountry").append(listItem);
                   index++;
               })
 }
       })

  <div class="country">
            <span class="pull-left mar-top-5">Country</span>
            <asp:DropDownList ID="filterCountry" runat="server" ClientIDMode="Static" CssClass="form-control pull-right country_drop_down"></asp:DropDownList>
           
        </div>
<div class="go">           
            <asp:Button runat="server" CssClass="btn btn-primary" ID="btnSubmit" OnClick="btnSubmit_Click" Text="Go" />
        </div>

 protected void btnSubmit_Click(object sender, EventArgs e)
        {
           
        ViewState["Query"]) = null;
//            BindReapterMain(Convert.ToString(ViewState["Query"]));
        }