Teacher:Given a scientific passage and an answer, generate a question for the given answer.
Teacher: Now, understand the problem? Solve this instance: Passage: Air pollution may also cause acid rain. This is rain that is more acidic (has a lower pH) than normal rain. Acids form in the atmosphere when nitrogen and sulfur oxides mix with water in air. Nitrogen and sulfur oxides come mainly from motor vehicle exhaust and coal burning. Answer: air pollution
Student:
Acid rain may be caused by what?