Answer the following question: You are presented with the quiz "when is fog common?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "cold fronts cause thunderstorms as they pass by" and "fog is common in the cold air ahead of the front". So, what's the best answer to the question?

Options: -- summer end; -- before thunderstorms; -- The area was once underwater; -- dry and hot; -- It used to be covered by water; -- never; -- outer space; -- under water;
Answer:
before thunderstorms