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