A specific location in America produces electricity only from fossil fuels and geothermal energy. The table below shows the amount of methane in the atmosphere of the location from 1960 to 2000.

Year 1960 1970 1980 1990 2000
Methane concentration (in parts per million by volume) 124 118 269 185 131

Based on the table, which of these conclusions is most likely correct about the period between 1980 and 1990?
Electricity was generated only from fossil fuels.
Electricity was generated only from geothermal energy.
Several fossil fuel units were replaced by geothermal units.
Several geothermal units were replaced by fossil fuel units.