Removing xml namespaces in C# restful web service returned string

I am having restful API which is written in C# using dot net framework 4.5..Currently its working alright... i'm returning a result after a JSON conversion.. I'm expecting a pure JSON result... which i'm not getting currently.. I'm expecting simple solution to omit the string XMLNS at the root element where i return the JSON... Result i'am getting:

My code :

public String GetAllSalesInvoices(string customer_id, string Startdate, string Enddate)
    {
        System.Web.Script.Serialization.JavaScriptSerializer serializer = new System.Web.Script.Serialization.JavaScriptSerializer();

        string query = "SELECT * FROM sales_invoice WHERE customer_id =" + customer_id + " AND invoice_date BETWEEN '" + Startdate + "' AND '" + Enddate + "'";
        DataSet ds = conObj.execQuery(query);
        DataTable dt = new DataTable();
        dt = ds.Tables[0];


        List<sales_invoice> result = new List<sales_invoice>();

        foreach (DataRow dr in dt.Rows)
        {
            sales_invoice inv = new sales_invoice()
            {
                Invoice_id = Convert.ToInt32(dr["invoice_id"]),
                Invoice_date = Convert.ToString(dr["invoice_date"].ToString()),
                Customer_id = Convert.ToInt32(dr["customer_id"]),
                Product_id = Convert.ToInt32((dr["product_id"])),
                Time = Convert.ToString((dr["time"]).ToString()),
                Quantity = Convert.ToInt32((dr["quantity"])),
                Unit_of_measure = Convert.ToString(dr["unit_of_measure"]),
                Product_price = Convert.ToInt32((dr["product_price"])),
                Sub_total = Convert.ToInt32((dr["sub_total"])),
            };
            result.Add(inv);
        }
        string json=serializer.Serialize(result);
        return json;
}

Thanks