Avoid Drop Down List refresh using AJAX

ASPX Page
<script type="text/javascript">

        $(document).ready(function () {
            GetItemName();
        })

        function GetItemName() {
            var pageUrl = '<%=ResolveUrl("/Forms/MaterialRequest.aspx")%>';
            var projectStatus = $("#<%=ddlCategory.ClientID%>").val();
            debugger;
            $.ajax({
                type: "POST",
                url: pageUrl + '/getItemName',
                data: '{ItemId:' + projectStatus + '}',
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                success: function OnlocationsPopulated(dvList) {
                    $("#<%=ddlItemName.ClientID%>").empty();
                    if (dvList.d.length > 0) {
                        $("#<%=ddlItemName.ClientID%>").removeAttr("disabled");
                        for (i = 0; dvList.d.length > i; i++) {
                            $("#<%=ddlItemName.ClientID%>").append("<option value=" + dvList.d[i].Value + ">" + dvList.d[i].Text + " </option>");
                        }
                    }
                    else {
                        $("#<%=ddlItemName.ClientID%>").empty().append('<option selected="selected" value="0">--Select--<option>');
                    }
                }
            });
        }
    </script>

Coad behind
  [System.Web.Services.WebMethod]
        public static ArrayList getItemName(string ItemId)
        {
            string id = ItemId;
            ArrayList dvList = new ArrayList();
            dvList = BLL.Sales.clsMaterialRequest.GetItemDetails1(ItemId);
            return dvList;
        }