Allow only single RadioButton selection from multiple RadioButtons in ASP.Net

I have 3 radio buttons if i click need to check one and show the semester as 1 and 2

<tr><td>
    <asp:Label ID="Lafbel6" runat="server" Text="Year" ForeColor="BLACK"></asp:Label></td><td>
    <asp:RadioButton ID="r1" runat="server" Text="Ist Year" AutoPostBack="True"  oncheckedchanged="r1_CheckedChanged" />
    <asp:RadioButton ID="r2" runat="server" Text="IInd Year" AutoPostBack="True" oncheckedchanged="r2_CheckedChanged" />
    <asp:RadioButton ID="r3" runat="server" Text="IIIrd Year" AutoPostBack="True" oncheckedchanged="r3_CheckedChanged" /><br />
</td></tr>   

<tr>
    <td>
        <asp:Label ID="Label7" runat="server" Text="Semester" ForeColor="BLACK"></asp:Label>
    </td>
    <td>
        <asp:CheckBox ID="c1" runat="server" Text="I " />
        <asp:CheckBox ID="c2" runat="server" Text="II" />
        <asp:CheckBox ID="c3" runat="server" Text="III" />
        <asp:CheckBox ID="c4" runat="server" Text="IV" />
        <asp:CheckBox ID="c5" runat="server" Text="V" />
        <asp:CheckBox ID="c6" runat="server" Text="VI" />
    </td>
</tr>