to insulate the electrical wire to keep the electrical wire warm to make the electrical wire stronger to make the electrical wire more flexible
I gave my students this multiple choice question: Which of the following best explains why electrical wires are usually covered with plastic or rubber?

Only one answer is correct among these 4 choices:
- to insulate the electrical wire
- to keep the electrical wire warm
- to make the electrical wire stronger
- to make the electrical wire more flexible

Could you tell me which one is correct?

to insulate the electrical wire