45. In order for blood to always flow unidirectionally through a closed circulatory system, the_______.
(A) blood vessels farthest from the heart must have valves
(B) capillaries must have a thick endothelium
(C) arteries must be elastic
(D) pressure in all vessels must be equal
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統計: A(1), B(0), C(0), D(2), E(0) #3056708
統計: A(1), B(0), C(0), D(2), E(0) #3056708