Over the past 60 years, the level of carbon dioxide in the atmosphere has been continually increasing. Many scientists hypothesize that this increase has led to an imbalance in the carbon cycle and a climatic trend known as global warming.

In which of the following ways could humans help the carbon cycle return to normal?

Cap volcanoes so that they do not release more carbon dioxide


cut down trees and other large plants that depend on carbon dioxide


reduce the amount of fossil fuels they combust


pump nitrogen and oxygen into the air to help rebalance the atmosphere