Question

early organisms turned ______ into oxygen, making life suitable for other organisms.

a. sulfur
b. silicon
c. nitrogen
d. carbon dioxide

Explanation:

Early photosynthetic organisms (like cyanobacteria) performed photosynthesis, which converts carbon dioxide ($CO_2$) and water into glucose and oxygen. Sulfur, silicon, and nitrogen are not the primary substrates for oxygen production in this context. So the correct option is D.

Answer:

D. carbon dioxide