It could possibly be a stroke.
The answer would be B. Only during periods of stress. When you are stressed out you are most likely having a hard time relaxing. If you take deep breaths your breathing should slow down and your body will relax. D. Only when you've gotten enough sleep, has nothing to do with breathing, you don't need to take deep breaths after sleeping. C. No more than twice a day and A. Everyday, are both unnecessary, you don't need to take deep breaths everyday for no reason. Stress is the main reason people hyperventilate. They get stressed and they have varied symptoms. They can't breathe, have short amount of patience, etc. <span />
The second statement best explains the type of blood vessel and the function of these vessels' valves - <span>These are veins, which prevent the backflow of blood because the walls have little muscle to pump blood back to the heart.
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Veins are large blood vessels which carry blood back to the heart. <span>Veins have a system of valves to prevent back-flow.</span>
