How to keep Dropdownlist value same after refresh the page

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C#

ASP.NET

I am using three dropdowns, in first dropdown i bind data from sql. In second dropdown i also bind data from sql and the third dropdown is bind according to second dropdown so in second dropdown there is autopostback property true. I have one search button which fetch the result from database on click.

I want to keep all the Dropdownlist value same after my result show on the page. One thing i did is, at the time of button click i make the session and check it at page load when it is not null bind the dropdown value. Till this point it is working fine i am able to bind prevoius selected values.
This is in my master page and result are display in result page, which is using this masterpage.

aspx :

<asp:DropDownList ID="dd_category" runat="server" AutoPostBack="True"              
onselectedindexchanged="dd_category_SelectedIndexChanged">
</asp:DropDownList>

<asp:DropDownList ID="dd_subcategory" runat="server" ></asp:DropDownList>

<asp:Button ID="but_go" runat="server" Text="Go" onclick="but_go_Click"/>

page load :

if (!IsPostBack)
    {
        FillStates();
        FillCategory();
    }


if (Session["state"] != null)
      {
            dd_state.SelectedValue = (string) Session["state"];
      }

aspx.cs :

private void FillStates()
{
    SqlCommand cmd = new SqlCommand("sps_bindcountrystate", con);
    cmd.CommandType = CommandType.StoredProcedure;
    cmd.Parameters.AddWithValue("@flag", 2);
    cmd.Parameters.AddWithValue("@CountryID", 1);
    DataSet objDs = new DataSet();
    SqlDataAdapter dAdapter = new SqlDataAdapter();
    dAdapter.SelectCommand = cmd;
    con.Open();
    dAdapter.Fill(objDs);
    con.Close();
    if (objDs.Tables[0].Rows.Count > 0)
    {
        dd_state.DataSource = objDs.Tables[0];
        dd_state.DataTextField = "s_name";
        dd_state.DataValueField = "s_id";
        dd_state.DataBind();
        dd_state.Items.Insert(0, "Select State");
    }
}

protected void dd_category_SelectedIndexChanged(object sender, EventArgs e)
{
    int CategoryID = Convert.ToInt32(dd_category.SelectedValue);
    FillSubCategory(CategoryID);
}

private void FillCategory()
{
    SqlCommand cmd = new SqlCommand("sps_bindcategory", con);
    cmd.CommandType = CommandType.StoredProcedure;
    cmd.Parameters.AddWithValue("@flag", 1);
    cmd.Parameters.AddWithValue("@CategoryID", 0);
    DataSet objDs = new DataSet();
    SqlDataAdapter dAdapter = new SqlDataAdapter();
    dAdapter.SelectCommand = cmd;
    con.Open();
    dAdapter.Fill(objDs);
    con.Close();
    if (objDs.Tables[0].Rows.Count > 0)
    {
        dd_category.DataSource = objDs.Tables[0];
        dd_category.DataTextField = "category_name";
        dd_category.DataValueField = "category_id";
        dd_category.DataBind();
        dd_category.Items.Insert(0, "Select Category");
        dd_subcategory.Items.Insert(0, "Select Sub-Category");
    }
}

private void FillSubCategory(int CategoryID)
{
    SqlCommand cmd = new SqlCommand("sps_bindcategory", con);
    cmd.CommandType = CommandType.StoredProcedure;
    cmd.Parameters.AddWithValue("@flag", 2);
    cmd.Parameters.AddWithValue("@CategoryID", CategoryID);
    DataSet objDs = new DataSet();
    SqlDataAdapter dAdapter = new SqlDataAdapter();
    dAdapter.SelectCommand = cmd;
    con.Open();
    dAdapter.Fill(objDs);
    con.Close();
    if (objDs.Tables[0].Rows.Count > 0)
    {
        dd_subcategory.DataSource = objDs.Tables[0];
        dd_subcategory.DataTextField = "subcategory_name";
        dd_subcategory.DataValueField = "subcategory_id";
        dd_subcategory.DataBind();
        dd_subcategory.Items.Insert(0, "Select Sub-Category");
    }
}

protected void but_go_Click(object sender, EventArgs e)
{
    Session["state"] = dd_state.SelectedValue;
    Session["category"] = dd_category.SelectedValue;
    Session["subcategory"] = dd_subcategory.SelectedValue;
    Response.Redirect("Results.aspx");
}

Now problem is when i again choose some different value in first dropdown and try to choose different value in second dropdown, the second dropdown is having postback so it refreshes page and according to session state it changes my first dropdown value back to prevoius one.

Tell me what to do so that new selected value does'nt change due to page refresh.