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In terrestrial environments, gravity places
special demands on the cardiovascular systems of
animals. Gravitational pressure can cause blood to
pool in the lower regions of the body, making it
diffi cult to circulate blood to critical organs such as
the brain. Terrestrial snakes, in particular, exhibit
adaptations that aid in circulating blood against the
force of gravity.
The problem confronting terrestrial snakes is best
illustrated by what happens to sea snakes when
removed from their supportive medium. Because the
vertical pressure gradients within the blood vessels
are counteracted by similar pressure gradients in the
surrounding water, the distribution of blood
throughout the body of sea snakes remains about
the same regardless of their orientation in space,
provided they remain in the ocean. When removed
from the water and tilted at various angles with the
head up, however, blood pressure at their midpoint
drops signifi cantly, and at brain level falls to zero.
That many terrestrial snakes in similar spatial
orientations do not experience this kind of circulatory
failure suggests that certain adaptations enable them
to regulate blood pressure more effectively in those
orientations.
One such adaptation is the closer proximity of the
terrestrial snake’s heart to its head, which helps to
ensure circulation to the brain, regardless of the
snake’s orientation in space. The heart of sea snakes
can be located near the middle of the body, a
position that minimizes the work entailed in
circulating blood to both extremities. In arboreal
snakes, however, which dwell in trees and often
assume a vertical posture, the average distance
from the heart to the head can be as little as 15
percent of overall body length. Such a location
requires that blood circulated to the tail of the
snake travel a greater distance back to the heart,
a problem solved by another adaptation. When
climbing, arboreal snakes often pause
momentarily to wiggle their bodies, causing waves
of muscle contraction that advance from the lower
torso to the head. By compressing the veins and
forcing blood forward, these contractions
apparently improve the fl ow of venous blood
returning to the heart.

It can be inferred from the passage that which of the
following is true of species of terrestrial snakes that
often need to assume a vertical posture?
(A) They are more likely to be susceptible to
circulatory failure in vertical postures than are
sea snakes.
(B) Their hearts are less likely to be located at the
midpoint of their bodies than is the case with
sea snakes.
(C) They cannot counteract the pooling of blood in
lower regions of their bodies as effectively as
sea snakes can.
(D) The blood pressure at their midpoint decreases
signifi cantly when they are tilted with their
heads up.
(E) They are unable to rely on muscle contractions
to move venous blood from the lower torso to
the head.

the OA is B.

But I cannot understand where in the passage it states that the heart of sea snakes is located at the midpoint of their body?