applying too much fertilizer to agricultural fields operation of coal-fired electric generating plants a hot summer day too much rain
I am hesitating between 4 options to answer the following question, which option should I choose?
Question: Which factor can increase the amount of sulfur dioxide in the air?
Possibilities:
- applying too much fertilizer to agricultural fields
- operation of coal-fired electric generating plants
- a hot summer day
- too much rain

operation of coal-fired electric generating plants