problem in ajax control

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.NET3.5

.NET

Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %><br />
<br />
<pre lang="xml"><asp:ToolkitScriptManager ID="ScriptManager1" runat="server" />
<asp:TextBox ID="txtDOB" runat="server" />
<asp:CalendarExtender ID="c1" runat="server" TargetControlID="txtDOB" /></pre><br />
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plese guide me........................