higher levels of oxygen in the atmosphere lower levels of ozone in the atmosphere higher levels of carbon dioxide in the atmosphere lower levels of nitrogen in the atmosphere
I gave my students this multiple choice question: The average temperature of Earth's surface has increased approximately 1°C in the past century. Which of the following can best explain this increase in temperature?

Only one answer is correct among these 4 choices:
- higher levels of oxygen in the atmosphere
- lower levels of ozone in the atmosphere
- higher levels of carbon dioxide in the atmosphere
- lower levels of nitrogen in the atmosphere

Could you tell me which one is correct?

higher levels of carbon dioxide in the atmosphere