auto complete extender Textbox Problem?

I have two text box my problem when I change country in textbox city textbox will be empty
Country : <asp:TextBox ID="txtCountry" runat="server" onkeyup="clear()" onChange="$find('AutoCompleteExtender2').set_contextKey(this.value);"></asp:TextBox> <cc1:AutoCompleteExtender ID="txtCountry_AutoCompleteExtender1" BehaviorID="AutoCompleteExtender1" runat="server" TargetControlID="txtCountry" ServiceMethod="GetCountriesList" ServicePath="~/services/JobPortalServices.asmx" CompletionInterval="10" CompletionSetCount="20" UseContextKey="true" MinimumPrefixLength="1" DelimiterCharacters="; ," EnableCaching="false" FirstRowSelected="true" CompletionListCssClass="completionList" CompletionListHighlightedItemCssClass="itemHighlighted" CompletionListItemCssClass="listItem"> </cc1:AutoCompleteExtender> City : <asp:TextBox ID="txtCity" runat="server" onChange="empty();"&g ...