Which of the following would NOT be considered a concerted activity? A. members of a union that meet to discuss problems with working conditions at their workplace B. employees who are not members of a union that meet to discuss problems with working conditions at their workplace C. a single employee that writes to a supervisor complaining about the refusal to grant her vacation time for the exact period of time she requested D. a single employee that writes to a supervisor complaining about the frequently malfunctioning air conditioning and extreme indoor heat at an assembly plant