What is friction? A. f net= ma B. the normal force C. the wearing down of surfaces D. forces opposing the motion of surfaces in contact with each other

D. Forces opposing the motion of surfaces in contact with each other

Explanation:

Friction moves in the opposite direction of the object.

I think the correct answer would be D. The tap water in the experiment is one the three test conditions of the independent variable, the type of water. The independent variable in a experiment is the one being manipulated or the one being changed. In this case, it is the type of water.