How to get the values of ASP.NET radio button inside ASP.NET repeater

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C#

ASP.NET

repeater

I am using asp.net repeater in the web application in that i am having four radio buttons and also fetching the data from the database to the repeater and radio buttons as well. Actual my problem is I am designing online examination site for which i am getting questions and options from the database but only thing when submitting the form it is considering first question fine but when comes to second question it is taking only first questions to second question and so on. How to solve this problem... Please suggest me where i am doing wrong.

What I have tried:

<div>
    <asp:Repeater ID="QuestionRepeater" runat="server">
        <HeaderTemplate>
            <table>
                <tr>
                    <td>Online Examination</td>
                </tr>
        </HeaderTemplate>
        <SeparatorTemplate>
                <tr>
                    <td>
                        <br />
                    </td>
                </tr>
        </SeparatorTemplate>
        <ItemTemplate>
                <tr>
                    <td>
                        <%#Eval("QuestionNumber") %>
                        <%#Eval("Question") %>
                    </td>
                </tr>
                <tr>
                    <td>
                        <asp:RadioButton runat="server" ID="rb1" GroupName="Rb_Choice" OnCheckedChanged="CheckChanged" Text='<%# Bind("ChoiceA") %>'></asp:RadioButton>
                    </td>
                    <td>
                        <asp:RadioButton runat="server" ID="rb2" GroupName="Rb_Choice" OnCheckedChanged="CheckChanged" Text='<%# Bind("ChoiceB") %>'></asp:RadioButton>
                    </td>
                </tr>
                <tr>
                    <td>
                        <asp:RadioButton runat="server" ID="rb3" GroupName="Rb_Choice" OnCheckedChanged="CheckChanged" Text='<%# Bind("ChoiceC") %>'></asp:RadioButton>
                    </td>
                    <td>
                        <asp:RadioButton runat="server" ID="rb4" GroupName="Rb_Choice" OnCheckedChanged="CheckChanged" Text='<%# Bind("ChoiceD") %>'></asp:RadioButton>
                    </td>
                 </tr>
        </ItemTemplate>
        <FooterTemplate>
            </table>
        </FooterTemplate>
    </asp:Repeater>
</div>

CodeBehind:

foreach (RepeaterItem rp in QuestionRepeater.Items)
{
    while (dr.Read())
    {
        if (rp.ItemType == ListItemType.Item || rp.ItemType == ListItemType.AlternatingItem)
        {
            a.QuestionNumber = Convert.ToInt32(dr["QuestionNumber"]);
            var rb1 = rp.FindControl("rb1") as RadioButton;
            var rb2 = rp.FindControl("rb2") as RadioButton;
            var rb3 = rp.FindControl("rb3") as RadioButton;
            var rb4 = rp.FindControl("rb4") as RadioButton;
        }
    }
}