can't references System.Data libraries

Question

Dear codemasters Good day to you all, I have an application written in vb.net Asp.net I want to add some features but currently i am writing in C# so I add a C# web form all is working I need to add a class to app_code then I found out that I can't add System.Data and System.Data.Sqlclient please any idea on how to add these libraries. thanks