Normal heart sounds are known as 'lub' and 'dub.' These sounds can be heart with a stethoscope as blood flows through the heart. According to the model of the heart sound cycle, what most likely causes the sound known as 'lub'? A) The 'lub' sound is caused as the atrioventricular valves close. B) The 'lub' sound is caused as the atrioventricular valves open. C) The 'lub' sound is caused as the semilunar valves close. D) The 'lub' sound is caused as the semilunar valves open