Comparing the Two Images Below -- the Difference in Emissions

Comparing the two images below -- the difference in emissions between the two plants below is MOST LIKELY due to:

A) a reduction in the combustion temperature of the fuel used.
B) conversion to solar energy as the energy source.
C) the use of a catalytic converter to reduce carbon monoxide emissions.
D) the use of an electrostatic precipitator to remove particulates.