Problem In Rating ajax toolkit

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C#

ASP.NET

Ajax

ViewState

I have been working on ajax control (Rating) in this when i am filling 3 star value is stored in viewstate and then on as same page i am using page1 and page2 value is moving from 1 st page to another page bt when i am clickning on previous button then answer sshow me 3 but rating star is not checked how to do this

protected void Rating1_Changed(object sender, AjaxControlToolkit.RatingEventArgs e)
    {
        Evaluate_Rating1(int.Parse(e.Value));

    }
  public void Evaluate_Rating1(int value)
    {
        lblAnswer1.Text = EvaluateRating(value, Rating1.MaxRating, RATING1_MIN, Rating1.MaxRating);
        ViewState["ans1"] = lblAnswer1.Text;
    }

  public static string EvaluateRating(int value, int maximalValue, int minimumRange, int maximumRange)
    {
        int stepDelta = (minimumRange == 0) ? 1 : 0;
        double delta = (double)(maximumRange - minimumRange) / (maximalValue - 1);
        double result = delta * value - delta * stepDelta;
        return FormatResult(result);
    }
 public static string FormatResult(double value)
    {
        return String.Format("{0:g}", value);
    }
on previous button i am doing this
protected void btnPrevious_Click(object sender, EventArgs e)
    {
        string s = Convert.ToString(ViewState["answerid"]);
        if (hndPageNo.Value != "")
        {
            Int32 pageNo = Convert.ToInt32(hndPageNo.Value);
           
            lblPageNo.Text = "Page No : " + (pageNo-1).ToString();       
       
            getQuestion(0);
        }
        DataTable dt;
        if (ViewState["studentOptionDetail"] == null)
        {
            dt = createOptionTable();
        }
        else
        {
            dt = (DataTable)ViewState["studentOptionDetail"];
        }
        DataRow dr = dt.NewRow();
        dr["studentID"] = 1;
        dr["questionID"] = hdnQuestion1.Value;   

       Evaluate_Rating1(int.Parse(s));
  
        dr["questionText"] = lblQuestion1.Text;
        dr["optionText"] = lblOptions1.Text;
  
        dt.Rows.Add(dr);
        ViewState["studentOptionDetail"] = dt;