reference to assembly 'System.Web.Extensions, Version=4.0.0.0

Trying to build a simple web control using the radGrid contol.
*added telerik.web.ui.dll to vs toolbox controls, add the scriptmanager , and radgrid controls,
When I try to build I receive error below

Error    1    The type 'System.Web.UI.ScriptManager' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.    C:\Program Files (x86)\telerik\Sitefinity 4.0\Projects\ceiwindows\MyControls\WebUserControl1.ascx.designer.cs    22    67    SitefinityWebApp

I have a reference to the ajax toolkit dll that is included in the libraries with sitefinity 4.
the assembly name of the telerik controls are Telerik.Web.UI(2010.3.1109.40)

My issue is fixed although I'm not sure what it was that resolved problem. For what it is worth Here is what I did.

Changed target .net to 3.5
closed  solution
reopened solution
ran build
changed back to target .net 4
closed solution
reopened solution
removed and re-added reference to ajax toolkit dll
ran build
error stopped being thrown.

Really Nice Answer....My Problem also resolved ...
I was also getting the Same Error when i have change the Framework from 3.5 to 4.0 for VS2010.
My Error was:-
The type 'System.Web.UI.ScriptManager' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.

I applied the same process, below given
-----------------------------------------------------------------------------------------
> First Change the .net target framework back from 4.0 to to 3.5 
> close the  solution
> reopen the solution 
> build your app (It may show many errors, don't worry)
> again Change the .net target framework back from 3.5 to 4.0 
> close the solution
> reopen the solution
> removed the reference of AjaxControlToolkit.dll
> again add the reference of AjaxControlToolkit.dll(Latest for 4.0 version i.e. for VS2010)
> build your app
> error stopped being thrown. 

Hello,

Thank you for sharing the exacts steps you took with the community. 

Regards,
Atanas Valchev
Telerik

After 1 spending 1 week trying to fix that problem that came up when I deployed an app to azure, the problem solved when i copied System.Web.Extensions.dll from another pc and pasted to mine in the 

C:\Windows\Microsoft.NET\assembly\GAC_MSIL\System.Web.Extensions\v4.0_4.0.0.0__31bf3856ad364e35 

directory.

Hi Evdokia..you save me