The graph illustrates how pressure varies in the left atrium and the left ventricle over one heartbeat. At time $X$, what is the condition of the valves in the heart?
- Aleft atrio-ventricular valve (bicuspid) closed; semi-lunar valve (in aorta) closed
- Bleft atrio-ventricular valve (bicuspid) closed; semi-lunar valve (in aorta) open
- Cleft atrio-ventricular valve (bicuspid) open; semi-lunar valve (in aorta) closed
- Dleft atrio-ventricular valve (bicuspid) open; semi-lunar valve (in aorta) open