code: def drive_rotational_speed_rpm(self):
          
          drv_rot_speed_rpm = set()
          for member in self.get_members():
              if member.rotational_speed_rpm is not None:
                  drv_rot_speed_rpm.add(member.rotational_speed_rpm)
          return drv_rot_speed_rpm
docs: Gets the set of rotational speed of the HDD drives