who is eric cartman’s father?

http://www.southparkzone.com/episodes/113/Cartmans-Mom-is-a-Dirty-Slut.html

http://www.southparkzone.com/episodes/202/Cartmans-Mom-is-Still-a-Dirty-Slut.html

When does she decide to tell eric the truth?
http://www.southparkstudios.com/clips/103398/kennys-sacrifice