向解决方案添加启用AJAX的WCF服务会引发错误

I am just trying to learn Ajax enabled WCF service, when I try to create a new web application and add new item "AJAX-enabled WCF Service" I get an error message that "The extension of type 'System.ServiceModel.Configuration.WebScriptEnablingElement, System.ServiceModel.Web, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' is not registered in the extension collection 'behaviorExtensions'.I am just trying to learn Ajax enabled WCF serv