Question

the relationship between humans and animals in which they exchange o₂ and co₂ is call the ___.
environment
water cycle
human system
carbon dioxide cycle
ecosystem

Explanation:

An ecosystem is a community of living organisms (humans and animals here) interacting with each other and their physical environment, which includes gas - exchange like $O_2$ and $CO_2$. The environment is just the surroundings, the water cycle involves water movement, the human system is about human - specific physiological etc. systems, and the carbon - dioxide cycle is mainly about carbon - dioxide movement in nature not the overall gas - exchange relationship in the context of the whole living community.

Answer:

E. ecosystem