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Oxygen is the most precious gas known to->by man. It makes up about one-fifth of an -> the air around us. Man cannot live with->without oxygen. When we breath->breathe,we take away air through the nose. The air is warmer and clean->cleaner while passing through the air passages on our bodies->body. In the lungs, oxygen pass->passes into the bloodstream. The heart pumps the bloods->blood with oxygen to the whole body. The heart works hardily->hard. It works even when we are sleeping,unlike the another->other organs in the body which slow down or rest.
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