I’m unsure of the answer, but it is not b.
The circulation system for mammals is very complex
<span>I am pretty sure the correct answer is C.
It makes the most sense.
Please correct me if I'm wrong.</span>
Earth's atmosphere is made up of several different elements,
nitrogen - 78%
oxygen - 21%
argon - 0.9%
carbon dioxide - 0.03%
and lastly water vapor with 0.0-4.0%
