21. A cross-country skier is struggling to get up a hill, so you offer to help. As the skier passes you, you reach out with your hand and exert an uphill force of 80 N (18 pounds) on the skier. When you do this, the skier exerts
- a downhill force of 80 N on you.
- a downhill force of somewhat more than 80 N on you.
- a downhill force of somewhat less than 80 N on you.
- no downhill force on you at all.