Create child list items under parent list item in ASP.net

public partial class Default : System.Web.UI.Page
{
    // First DropdownList items
    protected string[] Movies ={ "select a movie", "Movie_1", "Movie_2", "Movie_3" };
    // Second DropdownList items. One among these to be choosen.
    protected string[] Movie_1={"Theatre_1","Theatre_2","Theatre_3"};
    protected string[] Movie_2 ={ "Theatre_4", "Theatre_5" };
    protected string[] Movie_3 ={ "Theatre_5", "Theatre_6", "Theatre_7" };
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            DropDownList1.DataSource = Movies;
            DropDownList1.DataBind();
        }
    }
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (DropDownList1.SelectedIndex == 1)
        {
            DropDownList2.DataSource = Movie_1;
            DropDownList2.DataBind();
        }
        if (DropDownList1.SelectedIndex == 2)
        {
            DropDownList2.DataSource = Movie_2;
            DropDownList2.DataBind();
        }
        if (DropDownList1.SelectedIndex == 3)
        {
            DropDownList2.DataSource = Movie_3;
            DropDownList2.DataBind();
        }
    }
}