Studies show that volcanoes emit about 130 to 380 million metric tons of carbon dioxide (CO2) per year. ​
Which question can best help scientists determine if either volcanic eruptions or human activities are the main cause of the increasing CO2 levels shown in Graph 2?
A
How many active volcanoes are there on Earth?
B
Is there more carbon in limestone and rock deposits or in coal, oil, and gas deposits?
C
Does the rate of combustion for fossil fuels occur faster than a volcanic eruption?
D
How has the emission of greenhouse gases from human activities changed over time?