Answer:
1.b opposite
Frictional force offers the resistance to the applied force opposing its motion. Thus, it always acts in the direction opposite to that of the object in motion. If force is applied to the left, then friction acts in the right.
2.b.gravity
Gravity, also called gravitation, in mechanics, the universal force of attraction acting between all matter.
3.a.gravity
Gravity is a force that pulls objects down. This force pulls all objects to the ground with the same acceleration.
4.a.friction
friction
It turns into heat: the brake blocks can get incredibly hot! No matter how fast you go, there comes a time when you need to stop. Brakes on a bicycle work using friction (the rubbing force between two things that slide past one another while they're touching).
