What is the independent variable in this experiment?

A group of scientists studied the environmental impact of internal combustion engines burning hydrocarbon fuels. The scientists equipped four vehicles with devices to capture and measure particulate emissions. One vehicle burned diesel fuel, one burned ordinary gasoline, one burned a gasoline/ethanol mixture, and... show more A group of scientists studied the environmental impact of internal combustion engines burning hydrocarbon fuels. The scientists equipped four vehicles with devices to capture and measure particulate emissions. One vehicle burned diesel fuel, one burned ordinary gasoline, one burned a gasoline/ethanol mixture, and one burned natural gas. The four vehicles had equal masses and carried identical cargo. The scientists drove each vehicle 400 kilometers, recording the volume of fuel burned and the quantity of particulate emissions generated.

What is the independent variable in this experiment?
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