Which of the Following Would NOT Be Considered a Concerted

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