Why is air quality important for growing crops?

Less tilling thanks to genetically modified herbicide-tolerant crops also means that farmers spend less time on their tractors, using less fuel and reducing carbon emissions.  Conservation tillage enabled by genetically modified crops has reduced greenhouse gas emissions on farms globally and can help farmers adapt to and mitigate climate change.  In fact, in 2014 5.2 billion pounds of atmospheric carbon dioxide emissions were reduced by conservation tillage and decreased fuel use made possible by genetically modified crops.  That’s equal to removing nearly 10 million cars from roads for one year. 

According to the USDA, adopting conservation tillage can save at least 3.5 gallons of fuel per acre for farmers who would spend less time on their tractors, reducing emissions.  If all of the corn planted in the U.S. (non-genetically modified and genetically modified combined) in 2015 was grown with conservation tillage methods, nearly 308 million gallons of fuel would be saved, equivalent to preventing 6.9 billion pounds of carbon emissions. 

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."