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

getting error

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

 

 

change:
<ajaxToolkit:ToolkitScriptManager ID="ScriptManager1" runat="server" />

 

to: <asp:ScriptManager ID="ScriptManager1" runat="server" />