Besides friction or heat, what is created by rubbing two things together?

Often, we can cause the exchange of electrons, which creates charge.

For example, rubbing a rubber comb with a wool cloth will create charge on the comb, and it will attract small bits of paper.

Friction is the answer

The correct answer is b

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.