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 difficult 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 significantly, 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 flow of venous blood returning to the heart.
In the passage, the author is primarily concerned with doing which of the following?
Explaining adaptations that enable the terrestrial snake to cope with the effects of gravitational pressure on its circulatory system
Comparing the circulatory system of the sea snake with that of the terrestrial snake
Explaining why the circulatory system of the terrestrial snake is different from that of the sea snake
Pointing out features of the terrestrial snake's cardiovascular system that make it superior to that of the sea snake
Explaining how the sea snake is able to neutralize the effects of gravitational pressure on its circulatory system
E选项：解释海蛇如何抵消重力对循环系统的影响。这个选项文中提到了，是靠海水浮力（这不是海蛇的能力，be able to 在选项中给我们的理解是水蛇的某种自身能力抵消的重力），而且这不是作者的中心意思。