query about articalship

I want to know that on what basis the big firms like PWC ,KPMG, Batliboi & Co. select the students for Articleship.As I am a student of CA IPCC and I want to do my Articleship in these firms
so what is the minimum percentage required ,what kind of questions they ask in Interveiwand in which months they offer vacancies for Articleship,on what topics a student should have knowledge.
Hoping for a Immediate & best reply Thank You