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 he 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. 7. The primary purpose of the third paragraph is to 　　(A) Introduce a topic that is not discussed earlier in the passage 　　(B) Describe a more efficient method of achieving an effect discussed in the previous paragraph 　　(C) Draw a conclusion based on information elaborated in the previous paragraph 　　(D) Discuss two specific examples of phenomena mentioned at the end of the previous paragraph 　　(E) Introduce evidence that undermines a view reported earlier in the passage