You are presented with the question "Which situation can cause a thunderstorm?" and the following answer choices: 
- Fog rising over a lake
 - pressure differences.
 - Earth's axis of rotation is tilted
 - Setting up lightning rods
 - Cold air advancing on cold air
 - Cold air advancing on warm air
 - Changing the angle of the sun
 - fall moving to winter

Now knowing that cold fronts cause thunderstorms as they pass by and if cold air is advancing on warm air, the front is called a cold front, choose the best answer.


 

      Cold air advancing on warm air   