applying too much fertilizer to agricultural fields operation of coal-fired electric generating plants a hot summer day too much rain
I gave my students this multiple choice question: Which factor can increase the amount of sulfur dioxide in the air?

Only one answer is correct among these 4 choices:
- applying too much fertilizer to agricultural fields
- operation of coal-fired electric generating plants
- a hot summer day
- too much rain

Could you tell me which one is correct?

operation of coal-fired electric generating plants