A 60-kg girl weighs herself by standing on a scale in an elevator. What does the scale read when?
(a) The elevator is descending at a constant rate of 10 m/s;
(b) The elevator is descending at 10 m/s and gaining speed at a rate of 2 m/s2;
(c) The elevator is ascending at 10 m/s but its speed is decreasing by 2 m/s in each second?