You are presented with the question "when is fog common?" and the following answer choices: 
- summer end
 - before thunderstorms
 - The area was once underwater
 - dry and hot
 - It used to be covered by water
 - never
 - outer space
 - under water

Now knowing that cold fronts cause thunderstorms as they pass by and fog is common in the cold air ahead of the front, choose the best answer.


 

  before thunderstorms       