Am having error on Ajax auto complete code , AutoComplete search not working

The Auto Complete Code is not working on my page and it is showing error on visual studio when i run it

Here is the code

Error	30	The type 'System.Web.UI.ExtenderControl' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.	C:\Users\migold\Documents\Visual Studio 2013\WebSites\DebugSite\Master\Landing.master	324	

HTML Code

<div class=" text-right col-lg-3" style=""><asp:TextBox ID="txtSearch" runat="server" Placeholder="Search....." CssClass="form-control" Height="34px" AutoCompleteType="DisplayName" autofocus=""></asp:TextBox><asp:AutoCompleteExtender runat="server" ServicePath="~/WebService.asmx" ID="AutoCompleteExtender1"  ServiceMethod="GetSearch" TargetControlID="txtSearch"   MinimumPrefixLength="1" ></asp:AutoCompleteExtender></div>

Code On Web Services.cs

 public WebService () {

        //Uncomment the following line if using designed components 
        //InitializeComponent(); 
    }

    [WebMethod]
    public string[] GetSearch(string prefixText)
    {
        string sql = "Select * from User Where Name like @prefixText";
        SqlDataAdapter da = new SqlDataAdapter(sql, "Data Source=STEVE\\KONN;Initial Catalog=con;Integrated Security=True");
        da.SelectCommand.Parameters.Add("@prefixText", SqlDbType.VarChar, 100).Value = prefixText + "%";
        DataTable dt = new DataTable();
        da.Fill(dt);
        string[] items = new string[dt.Rows.Count];
        int i = 0;
        foreach (DataRow dr in dt.Rows)
        {
            items.SetValue(dr["Name"].ToString(), i);
            i++;
        }
        return items;
    }

    
}