Friction between sliding surfaces is caused by inertia, microwelds, the normal force, or gravity

Explanation:

Friction can be defined as the resistance of movement applied on a object, that's why is always in opposite direction of the movement.

However, the full definition is accomplish by analysing its equation:

Where is the coefficient of friction, which change from 0 to 1, depending on the material of the surface. refers to the normal force, which is the reaction of the weight, according with Newton's third law, its equal and opposite to the weight.

As you can see, friction depends directly on the normal force, the greater is this force, the greater will be the "friction force".

Therefore, the right answer is the third option.

it takes about 30 days for it to break down

when it gains or loses electrons