Answer:
a-it causes friction to increase
Step-by-step explanation:
Imagine you have a smooth sheet of ice and you slide a smooth box across it. It'll slide petty far because there's not much friction. The two smooth surfaces aren't "grabbing" each other very much.
Now try to slide that same box over gravel. It's not going to go nearly as far. It gets caught by all the little bits of rock sticking up and falls into the divots.
Another example would be to think of sliding two smooth pieces of paper across each other as opposed to trying to slide a set of velcro the same way.
Whenever you have to answer questions about conceptual friction, it helps to picture it big and in real world examples!