what is this how to avoid "ctl00"

following scrip i am using in asp.ne web application
this code will be write in my master page

 <script type="text/javascript">



        function IAmSelected(source, eventArgs) {



            $find('"AutoCompleteExtender3 ').set_contextKey(eventArgs.get_value());

            $find('"AutoCompleteExtender2 ').set_contextKey(eventArgs.get_value()); 



        }



    </script>





<table>



<tr>



<td>



<asp:TextBox ID="txt_city" runat="server"></asp:TextBox>



<asp:TextBoxWatermarkExtender ID="watermark1" runat="server" TargetControlID="txt_city" WatermarkText="Coimbatore"></asp:TextBoxWatermarkExtender>



<asp:AutoCompleteExtender ID="cityautocomplete" 



TargetControlID="txt_city"



 runat="server" 



  MinimumPrefixLength="1" 



    CompletionInterval="10"



    EnableCaching="true"



    ServiceMethod="GetCompletionList"



    OnClientItemSelected="Selectarea" 



 ServicePath="~/AutoComplete.asmx"></asp:AutoCompleteExtender>



</td>



<td>



<asp:TextBox ID="txt_Search" runat="server" Height="21px" Width="281px"></asp:TextBox>



<asp:AutoCompleteExtender ID="Searchautocomplete"  



TargetControlID="txt_Search"



 runat="server" 



  MinimumPrefixLength="1" 



    CompletionInterval="10"



    EnableCaching="true"



    ServiceMethod="GetSearchList"



 ServicePath="~/AutoComplete.asmx"></asp:AutoCompleteExtender>



<asp:TextBoxWatermarkExtender ID="TextBoxWatermarkExtender1" runat="server" TargetControlID="txt_Search" WatermarkText="Search Example - ICICI Bank"></asp:TextBoxWatermarkExtender>



</td>



<td>



<asp:TextBox ID="txt_Place" runat="server"> </asp:TextBox>



<asp:AutoCompleteExtender ID="AutoCompleteExtender1" 



TargetControlID="txt_Place"



 runat="server" 



  MinimumPrefixLength="1" 



    CompletionInterval="10"



    EnableCaching="true"



    ServiceMethod="GetAreaList"



 ServicePath="~/AutoComplete.asmx"></asp:AutoCompleteExtender>



<asp:TextBoxWatermarkExtender ID="TextBoxWatermarkExtender2" runat="server" TargetControlID="txt_Place" WatermarkText="Rspuram"></asp:TextBoxWatermarkExtender>

</td>



<td>



<asp:Button ID="btn_Search" runat="server" Text="Go" onclick="btn_Search_Click" />



</td>



</tr>

</table>

 <script type="text/javascript">



        function IAmSelected(source, eventArgs) {



            $find('"ctl00_AutoCompleteExtender3 ').set_contextKey(eventArgs.get_value());

            $find('"ctl00_AutoCompleteExtender2 ').set_contextKey(eventArgs.get_value());

 

        }



    </script>