Calculating percentage and displaying correct answer for online examination website in ASP.NET

protected void examsubmit_Click(object sender, EventArgs e) {

    DataTable dt = new DataTable();
    string sqlStatement = "SELECT * FROM [Biology]";



      SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["RegistrationConnectionString2"].ConnectionString);

        using (SqlCommand cmd = new SqlCommand(sqlStatement, con))
        {
            cmd.CommandType = CommandType.Text;
            using (SqlDataAdapter da = new SqlDataAdapter(cmd))
            {
                da.Fill(dt);

                if (dt.Rows.Count > 0)
                {
                    int index = 0;
                    foreach (GridViewRow row in grdquestions.Rows)
                    {
                        RadioButton rb1 = (RadioButton)row.FindControl("Option1");
                        RadioButton rb2 = (RadioButton)row.FindControl("Option2");
                        RadioButton rb3 = (RadioButton)row.FindControl("Option3");
                        RadioButton rb4 = (RadioButton)row.FindControl("Option4");
                        //
                        int marks = Convert.ToInt32(dt.Rows[index]["Marks"]);
                        if (rb1 != null || rb2 != null || rb3 != null || rb4 != null)

                            if (rb1.Text == dt.Rows[index]["Answer"].ToString())
                            {
                                //do something if matched

                                rb1.Checked = true;


                            }

                            else if (rb2.Text == dt.Rows[index]["Answer"].ToString())
                            {
                                //do something if matched

                                rb2.Checked = true;


                            }

                            else if (rb3.Text == dt.Rows[index]["Answer"].ToString())
                            {
                                //do something if matched

                                rb3.Checked = true;


                            }

                            else if (rb4.Text == dt.Rows[index]["Answer"].ToString())
                            {
                                //do something if matched

                                rb4.Checked = true;


                            }
                                index++;


                        int total =index * marks;
                        Label2.Text = "FinalScore is :" + total;


                          }


                        }

                    }

                }


            }