increased level of oxygen increased level of carbon dioxide decreased ability to support life decreased ability to transmit light
I gave my students this multiple choice question: Which of the following best describes a change in Earth's atmosphere made by early photosynthetic life?

Only one answer is correct among these 4 choices:
- increased level of oxygen
- increased level of carbon dioxide
- decreased ability to support life
- decreased ability to transmit light

Could you tell me which one is correct?

increased level of oxygen