In one year, researchers estimated that 5 billion tons of CO2 were emitted from cars, 20 billion tons of CO2 were emitted from industries, 20 billion tons of CO2 were emitted from energy plants, and 4 billion tons of CO2 were emitted from domestic usage. Assuming that these sources account for all major human-derived sources of CO2 pollution, what percentage of CO2 pollution can be attributed to domestic usage? (a) 2.3 percent (b) 3.0 percent (c) 8.2 percent (d) 22.5 percent