Zhang Nan is a staff member of an institute. His friends are doctors or professors. Wang Xiao is a staff member of the institute. Zhao Lin is a middle school classmate of Zhang Nan. If the above information is true, which of the following must be true? ["Wang Xiao is Zhang Nan's friend.", "Zhao Lin is not Zhang Nan's friend.", 'If Wang Xiao is a doctor, then Wang Xiao is a friend of Zhang Nan.', 'If Zhao Lin is a friend of Zhang Nan, then she is a doctor or professor.']