How to bind or populate data into a Dropdown List based on first or another Dropdown List in Asp.net C#

public partial class Default2 : System.Web.UI.Page
{
    SqlConnection con= new SqlConnection();

    protected void Page_Load(object sender, EventArgs e)
    {
        con.ConnectionString = "server=a2ul-pc;uid=sa;pwd=rascal;database=cgrt";

        drop_pcode = pcode;
        drop_fyyear = fyyear;


        if (!IsPostBack)
        {
            FillDropDownList();
        }
    }

    private void FillDropDownList()
    {
        DataSet ds = new DataSet();
        SqlDataAdapter myda = new SqlDataAdapter("select pcode FROM yearly", con);

        myda.Fill(ds);
        drop_pcode.DataSource = ds;
        drop_pcode.DataValueField = "Pcode";
        drop_pcode.DataBind();
        drop_pcode.Items.Insert(0, new ListItem("Select", "0"));
    }

    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        DataSet ds = new DataSet();
        SqlDataAdapter myda = new SqlDataAdapter("Select fyyear from yearly where pcode='"+drop_pcode.SelectedItem.Value+"'",con);
        myda.Fill(ds);
        drop_fyyear.DataSource = ds;
        drop_fyyear.DataValueField = "fyyear";
        drop_fyyear.DataBind();
        drop_fyyear.Items.Insert(0, new ListItem("Select", "0"));
    }
}