Please choose the correct answer to the question: The average temperature of the surface of Earth has increased approximately 1°C in the past century. Which reason best explains this increase in temperature?

Answer1: higher levels of oxygen in the atmosphere Answer2: lower levels of ozone in the atmosphere Answer3: higher levels of carbon dioxide in the atmosphere Answer4: lower levels of nitrogen in the atmosphere

Answer format: answer1/answer2/answer3/answer4
Answer: answer3