Ensuring only one item is selected in a nested radiobuttonlist

Hi i have a nested radiobuttonlist which is in a repeater, however what i am trying to do is ensure that when the items are returned only one item can be selected. The repeater returns a category and the subcategories. Therefore there could be 7 categories, and each one has 2 or 3 subcategories. Here is my code so far;

  <asp:Repeater ID="rptCategory" OnItemDataBound="rptCategory_OnItemDataBound" runat="server">
<ItemTemplate>
<tr>
<td class="Col1"><img src="images/spacer.gif" alt="" />
<%#DataBinder.Eval(Container.DataItem, "_CategoryName")%>
...