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

Fact 2: If cold air replaces warm air, it is called a cold front.

Given the two facts above, answer the question "What does cold air replacing warm air cause?" with the following options: 
- Greenhouse gases
 - Pollution
 - Dehydration
 - thunderstorms
 - steady barometer readings
 - hypothyroidism
 - constant air pressure
 - stagnant air temperature


 

    thunderstorms     