It was very obvious that the cofounder couldn't answer his own questions. The tech stack question has no definitive answer (do a quick Google search) and it is based on preference. I answered the C OOP question using structs and header files but he was not satisfied with my answer. I received 3 job offers from Fortune 500 tech companies after this interview, so I'm very knowledgable when it comes to coding interviews, but the cofounder obviously didn't know the answers to his own interview questions given his body language. He was looking for very cheap labor from people with senior level skills.