mutually exclusive checkbox could not run when select ?

hello sir ajax mutually exclusivecheckbox could not run  when is select onecheckbox and then select another checkbox then checkbox tick could not remove of previous when select second one  

here is my code

aspx

<asp:CheckBox ID = "chkMango" Text="Mango" runat="server" /><br />
<ajaxToolkit:MutuallyExclusiveCheckBoxExtender ID="meceMango" runat="server" TargetControlID = "chkMango" Key = "Fruit">
</ajaxToolkit:MutuallyExclusiveCheckBoxExtender>

<asp:CheckBox ID = "chkApple" Text="Apple" runat="server" /><br />
<ajaxToolkit:MutuallyExclusiveCheckBoxExtender ID="meceApple" runat="server" TargetControlID = "chkApple" Key = "Fruit">
</ajaxToolkit:MutuallyExclusiveCheckBoxExtender>

<asp:CheckBox ID = "chkBanana" Text="Banana" runat="server" /><br />
<ajaxToolkit:MutuallyExclusiveCheckBoxExtender ID="meceBanana" runat="server" TargetControlID = "chkBanana" Key = "Fruit">
</ajaxToolkit:MutuallyExclusiveCheckBoxExtender>

<asp:CheckBox ID = "chkGuava" Text="Guava" runat="server" /><br />
<ajaxToolkit:MutuallyExclusiveCheckBoxExtender ID="meceGuava" runat="server" TargetControlID = "chkGuava" Key = "Fruit">
</ajaxToolkit:MutuallyExclusiveCheckBoxExtender>

<asp:CheckBox ID = "chkOrange" Text="Orange" runat="server" /><br />
<ajaxToolkit:MutuallyExclusiveCheckBoxExtender ID="meceOrange" runat="server" TargetControlID = "chkOrange" Key = "Fruit">
</ajaxToolkit:MutuallyExclusiveCheckBoxExtender>