Answer: D
Explanation: Six people can push with more force than 5 people can. So, if 5 people can push the box up a 400-cm ramp, then certainly 6 people can do it also. Similarly, any ramp that is longer (less steep) than a 400-cm ramp will work, so they can handle the 800-cm ramp. However, the 200-cm ramp is too short and steep. In fact, the 200-cm ramp is half the distance of the 400-cm ramp, so it would take twice as many people (10 people) to push the box up that ramp. Six people is not enough
