Fact 1: Cold fronts cause thunderstorms as they pass by.

Fact 2: Thunderstorms and lightning Lightning often accompanies thunderstorms.

Given the two facts above, answer the question "What causes lightning?" with the following options: 
- dust
 - energy
 - cold fronts
 - sunlight
 - Energy.
 - scattered light
 - fire
 - heat energy


 

   cold fronts      