This chart gives data on greenhouse gas emissions in the United States from 1990 to 2013. Which questions would help clarify the evidence in the chart? What percentage of total greenhouse gas emissions is caused by natural sources? Should industries emitting carbon dioxide in huge amounts be fined or punished? Why are carbon dioxide emissions so much higher than other greenhouse gases? Why did the quantity of carbon dioxide emissions stay constant throughout the period? What is the chemical formula for methane?