The immune system, and the excretory system
(Honestly, all of them work to keep us 'healthy', but I believe these are the ones you're looking for)
Although, I'm not sure about the entirety of this question, when reviewing claims and evidence, check your sources to make sure they are reliable.
I promise to you I’m not lying the answer is D I just had this question
Answer:
chemicals in the biowater recalculate
Explanation:
A population would be an organism (the same species) living in an area