Solved: Could not load type

I came across following error while using AjaxToolKit 3.5, well it is a very common error I think, and I got the solution "Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'."