Error 1 The type 'System.Web.Compilation.WCFBuildProvider' is ambiguous: it could come from assembly 'C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Web.Extensions\v4.0_4.0.0.0__31bf3856ad364e35\System.Web.Extensions.dll' or from assembly 'C:\WINDOWS\assembly\GAC_MSIL\System.Web.Extensions\3.5.0.0__31bf3856ad364e35\System.Web.Extensions.dll'. Please specify the assembly explicitly in the type name. App_WebReferences/
How do I fix this one?