- To produce something means to form something
- A source of something emits / produces / generates / provides that something
- So, a source of something forms that something

- A source of something forms that something
- Sulfur dioxide is formed by the combustion of fossil fuels
- Therefore, the combustion of fossil fuels is a source of sulfur dioxide

Hence, the combustion of fossil fuels is a source of sulfur dioxide
[Question]Gaseous sulfur dioxide is a compound that combines with water in the atmosphere to form acid rain. What is the primary source of sulfur dioxide?
[Context]- To produce something means to form something 
- A source of something emits / produces / generates / provides that something 
- Sulfur dioxide is formed by the combustion of fossil fuels