Fact 1: Magnetism can cause objects to repel each other.

Fact 2: Opposite charges repel.

Given the two facts above, answer the question "What can cause objects to have opposite charges?" with the following options: 
- Temperature
 - frozen water
 - heat energy
 - magnetism
 - polar winds
 - ship captains
 - voltage
 - Light


 

    magnetism     