Raddropdowntree is not dropping down at runtime in asp.net web app using c#

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ASP.NET

C#3.5

Telerik

Hi Frnds, I am priya. i am struggling to get my head around raddropdowntree.
i used this in my asp.net web app using c#.

i added the following code
in aspx file
<%@ Register Assembly="Telerik.Web.UI" Namespace="Telerik.Web.UI" TagPrefix="telerik" %>

and

<telerik:RadScriptManager ID="RadScriptManager1" runat="server">
<telerik:RadDropDownTree runat="server" ID="RadDocument" Skin="Telerik" Width="250px"
DefaultMessage ="Choose a document" DefaultValue="0" DataFieldID="ID" DataFieldParentID="ParentID" DataTextField="Text">
<dropdownsettings height="140px">



in aspx.cs file

public DataTable GetData()
{
DataSet ds = new DataSet();
DataTable table = new DataTable();
table.Columns.Add("ID");
table.Columns.Add("ParentID");
table.Columns.Add("Value");
table.Columns.Add("Text");
table.Rows.Add(new String[] { "1", null, "Africa" });
table.Rows.Add(new String[] { "2", "1", "Egypt" });
table.Rows.Add(new String[] { "3", "2", "Cairo" });
table.Rows.Add(new String[] { "4", "1", "South Africa" });
table.Rows.Add(new String[] { "5", "4", "Cape Town" });
return table;
}

i attached data to raddropdowntree
protected void Page_Load(object sender, EventArgs e)
{
RadDocument.DataSource = GetData();
RadDocument.DataBind();
RadDocument.ExpandAllDropDownNodes();
}


but when i run the app, it is showing control alone with "choose a document" text. it is not dropping down when i click on it. please kindly help me.