Question

your skateboard begins to slow down when you roll off the sidewalk and across gravel. what is at work?
gravity
friction
balance
all of the above

Explanation:

Friction is a force that opposes motion. When the skateboard moves from the smooth sidewalk to the rough gravel, the increased friction between the skateboard wheels and the gravel surface causes the skateboard to slow down. Gravity acts vertically and doesn't directly cause the horizontal slow - down in this case. Balance is a state and not a force causing the slow - down.

Answer:

B. friction