Most vertebrates that live on land have two circuits for blood
with relatively separate pathways through the heart. One pathway,
called the systemic circuit, leaves the heart and travels to all
the parts of the body and then back to the heart. The second,
called the pulmonary circuit, travels from the heart to the
lungs and back. However, the amphibian heart is almost like an
inline pump that mixes blood from both circuits. Which statement
below best describes why this setup works in amphibians?
A. because the skin and lung surface are
both respiratory membranes, so blood in both circuits
replenish the oxygen supply
B. because the amphibian lives some of its life in water, which
has less oxygen than the air
C. because the larvae of amphibians use gills to carry out
respiration
D. because amphibians have a low metabolism and the heart has to
spend less energy to pump if there are fewer chambers