Answer:
The independent variable is the type of fuel
Explanation:
The independent variable in an experiment is the variable that is being changed in a scientific experiment. It is the cause of any of the observed outcomes of the experiment. The Independent variables is the variable that the experimenter changes to analyse its effect on the dependent variable(outcome).
In this case, the type of fuel (independent variable) is being changed and its effect on the amount of pollution (dependent variable) is observed.