Answer:
A. It is the only one that makes sense
Explanation:
Positive - Friction allows us to create heat in a desperate situation, like being lost in the woods. If we didn't have friction, we would probably freeze to death.
Negative - Friction can also cause unwanted fires, such as forest fires. If friction didn't exist, we wouldn't have these.
A. Equal and opposite
For every action, there is an equal and opposite reaction
How many stairs?
You can use this to find the work
U
W=mgh
And the power by
P=W/T