Mammals breathe with the help of a diaphragm.
Breathing stomach muscles press against the resistance of the diaphragm.
Mammals breathe with the help of something that presses against the stomach muscles.
What is pressed against when mammals breathe?

a) objects that have mass
b) the skin
c) an organism's body
d) an object
e) layers of fat
f) matter vibrating
g) a cuticle
h) stomach muscles