The graph illustrates how pressure varies in the left atrium, left ventricle and aorta throughout one cardiac cycle. Referring to the semilunar valve and atrioventricular valve, what is occurring at points 1, 2, 3 and 4?
- Asemilunar valve opens at 1, closes at 2; atrioventricular valve opens at 3, closes at 4
- Bsemilunar valve opens at 1, closes at 2; atrioventricular valve opens at 4, closes at 3
- Csemilunar valve opens at 2, closes at 3; atrioventricular valve opens at 1, closes at 4
- Dsemilunar valve opens at 2, closes at 3; atrioventricular valve opens at 4, closes at 1