12. Running on soft dry sand is exhausting, so you switch to running on hard wet sand. The hard wet sand removes less energy from you because

• it pushes up on your foot just as hard as your foot pushes on it, unlike the soft dry sand.
• it barely moves downward as you push downward on it, so you do almost zero work on it.
• it stops the downward motion of your foot faster and thus absorbs less of your momentum.
• it stops the downward motion of your foot faster and thus absorbs more of your momentum.