The first heart sound, "lubb," is caused by the . The first heart sound, "lubb," is caused by the . closing of the atrioventricular valves regurgitation of the atrioventricular valves closing of the semilunar valves blood flowing into the ventricles

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primary succession occurs in essentially lifeless areas regions in which the soil is incapable of sustaining life as a result of such factors as lava flows, newly formed sand dunes, or rocks left from a retreating glacier. secondary succession occurs