How to listbox items will be selected

protected void Page_Load(object sender, EventArgs e)
      {
          List<string> lst = new List<string>();
          lst.Add("one");
          lst.Add("Agrinational Insurance Company Inc");
          lst.Add("two");
          lst.Add("American Agri-Business Insurance Company");
          lst.Add("three");
          lst.Add("American Agricultural Insurance Company");
          lst.Add("four");
          ListBox1.SelectionMode = ListSelectionMode.Multiple;
          ListBox1.DataSource = lst;
          ListBox1.DataBind();

      }

      protected void btn_Click(object sender, EventArgs e)
      {
          string ss = "Agrinational Insurance Company Inc;American Agri-Business Insurance Company;American Agricultural Insurance Company";
          string[] items = ss.Split(';');
          foreach (ListItem item in ListBox1.Items)
          {
              if (items.Contains(item.Text))
                  item.Selected = true;
          }
      }