When a bicycle tire gets mud on it, the tire becomes slippery. what does the mud do to the tire? a. increases the force of gravity b. increases the force of friction c. it decreases the force of friction d. it decreases the force of gravity

The answer to this is C.

Question: When a bicycle tire gets mud on it, the tire becomes slippery. What does the mud do to the tire?

C. It decreases the force of friction.

First mud is not a solid. It is more of a slow liquid like corn syrup. And when you are caught in mud or trying to ride past the mud you loose friction with the ground.
You could also resemble friction with traction. In a car. If there is water & oil mixed on the road and you go past it at a high mph range. You will loose traction and start skipping against the mixed substance. This reduces your traction (friction) with the ground.

Final

C. It decreases the force of friction

The duration of a photographic flash is related to an rc time constantly

The independent variable would be the fertilizer as it is what the dependent variable (the growth of grass) depends on.