The main purpose of the system is to distribute blood throughout the body. A. circulatory B. respiratory C. digestive D. musculoskeletal

A, Circulatory

Explanation: Respiratory has to do with breathing, digestive has to do with things you eat, musculoskeletal has to do with the way your body moves

d) center of the mid-ocean ridge

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sinserly,

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many other transport proteins, often called pumps, move materials against a concentration gradient. active transport drives molecules across a membrane from a region of lower concentration to a region of higher concentration. this process, shown in figure 5.1, uses transport proteins powered by chemi- cal energy.