ASP.net select only one radio button in extra GridView columns

Hi

I am using the ASP.net markup code below for a GridView that has an extra
5 columns, each containing a radio button.
I want a user to only be able to select one radio button.
How do I do this? At the moment the user can select all 5 at the same time

&nbsp;<asp:GridView ID="GridView1" runat="server" CellPadding="4" 
            ForeColor="#333333" GridLines="None" style="margin-top: 0px">  
            <AlternatingRowStyle BackColor="White" ForeColor="#284775" />
            <Columns> 
                <asp:TemplateField>
                   <ItemTemplate>
                       <asp:RadioButton ID="RadioButton1" runat="server" />
                  </ItemTemplate>
               </asp:TemplateField> 
                      <asp:TemplateField>
                   <ItemTemplate>
                       <asp:RadioButton ID="RadioButton2" runat="server" />
                  </ItemTemplate>
               </asp:TemplateField> 
                      <asp:TemplateField>
                   <ItemTemplate>
                       <asp:RadioButton ID="RadioButton3" runat="server" />
                  </ItemTemplate>
               </asp:TemplateField> 
                      <asp:TemplateField>
                   <ItemTemplate>
                       <asp:RadioButton ID="RadioButton4" runat="server" />
                  </ItemTemplate>
               </asp:TemplateField> 
                      <asp:TemplateField>
                   <ItemTemplate>
                       <asp:RadioButton ID="RadioButton5" runat="server" />
                  </ItemTemplate>
               </asp:TemplateField> 
            </Columns> 
            <EditRowStyle BackColor="#999999" />
            <FooterStyle BackColor="#5D7B9D" ForeColor="White" Font-Bold="True" />  
            <HeaderStyle BackColor="#003366" Font-Bold="True" ForeColor="White" />  
            <PagerStyle BackColor="#284775" ForeColor="White" HorizontalAlign="Center" />  
            <RowStyle BackColor="#F7F6F3" ForeColor="#333333" />  
            <SelectedRowStyle BackColor="#E2DED6" Font-Bold="True" ForeColor="#333333" />  
            <SortedAscendingCellStyle BackColor="#E9E7E2" />  
            <SortedAscendingHeaderStyle BackColor="#506C8C" />  
            <SortedDescendingCellStyle BackColor="#FFFDF8" />  
            <SortedDescendingHeaderStyle BackColor="#6F8DAE" />  
        </asp:GridView>

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