Change value of arraylist on specific index

See more:

C#

I am developing Online Examination Application .Answers are fetching in radio button. if user click next button that will change question/Answers and also adding Arraylist from selected radion button

Problem is that on click previou button Array list Also Adding except adding it should b updating on previous index.

private void btnPrevious_Click(object sender, EventArgs e)
    {
        CountRating();
           if (i == ds.Tables[0].Rows.Count - 1 || i != 0)
        {
            i--;

            ShowData();
        }

        else
        {
            MessageBox.Show("no record found");
        }
    }

public void CountRating()
      {
          try
          {
              if (radioButton1.Checked == true)
              {
                  RB = radioButton1.Text;
                  radioButton1.Checked = false;
              }
              else if (radioButton2.Checked == true)
              {
                  RB = radioButton2.Text;
                  radioButton2.Checked = false;

              }
              else if (radioButton3.Checked == true)
              {
                  RB = radioButton3.Text;
                  radioButton3.Checked = false;
              }
              else if (radioButton4.Checked == true)
              {
                  RB = radioButton4.Text;
                  radioButton4.Checked = false;
              }
              if (RB == "Excellent")
              {
                  rating = 5;
              }
              else if (RB == "Yes")
              {
                  rating = 5;

              }
              else if (RB == "Good")
              {
                  rating = 4;
              }
              else if (RB == "Average")
              {
                  rating = 3;
              }
              else if (RB == "Fair" || RB == "No")
              {
                  rating = 1;
             }
              else
              {
                  MessageBox.Show("masla");
              }
          }
          catch (Exception ex)
          {
              MessageBox.Show(ex.Message);
          }
      }

public void ShowData()
        {
            key = Convert.ToInt32(ds.Tables[0].Rows[i]["qus_id"]);
            lblQ.Text = "Q. " + ds.Tables[0].Rows[i]["qus_id"].ToString();
            lblQues.Text = ds.Tables[0].Rows[i]["questions"].ToString();
            radioButton1.Text = ds.Tables[0].Rows[i]["Ans1"].ToString();
            radioButton2.Text = ds.Tables[0].Rows[i]["Ans2"].ToString();
            if (ds.Tables[0].Rows[i].IsNull("Ans3"))
            {

                radioButton3.Hide();
                radioButton4.Hide();

            }
            else
            {
                radioButton3.Show();
                radioButton4.Show();
                radioButton3.Text = ds.Tables[0].Rows[i]["Ans3"].ToString();
                radioButton4.Text = ds.Tables[0].Rows[i]["Ans4"].ToString();
            }
        }