to enhance the effectiveness of friction to increase the efficiency of the machine to provide an additional source of energy to produce more thermal energy on the machine
I gave my students this multiple choice question: Machines are often full of moving parts that rub against each other. In machines, a lubricant such as oil is often applied to these moving parts. Why is this oil necessary?

Only one answer is correct among these 4 choices:
- to enhance the effectiveness of friction
- to increase the efficiency of the machine
- to provide an additional source of energy
- to produce more thermal energy on the machine

Could you tell me which one is correct?

to increase the efficiency of the machine