<cc1:ToolkitScriptManager runat="server" ... /> and changin

Question

An error i am getting   

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

 

 

after changing the

 

 this one 

<cc1:ToolkitScriptManager runat="server" ... /> 

to this one 

<asp:ScriptManager runat="server" ... /> 

 

the calender in not showing

 

can  pl solved my problem