Many people may not know it, but the Australian Platypus is a mammal that can actually LAY eggs!
There is also Monotremes, who are also mammals, lay eggs and feed their young on milk.
Finally, the Echidna is a mammal who lays eggs, sort of like a bird, but isn't!
(All these animals feed their young on milk, not just the Monotremes!)
Interesting, right?
↑ ↑ ↑ Hope this helps! :D
The answer is...
C) Medulla Oblongata
(The brainstem is made up of the Medulla Oblongata, Pons, Mesencephalon.)
Answer:
increasing the amount of oxygen