System.Web. Extensions.dll assembly
This is the dll which comes with asp.net instillation package,which facilitate for AJAX functions..normaly it kept on
"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\v3.5" in .Net framework 3.5 but it differ from some .Net versions.
Normaly we place this in our application bin folder(for simplify the deployment)..but it's not reccomended.