answer is C
my question is why not B? is it because atria and ventricle contraction occur simultaneously?
Look at the final option, the semi-lunar valves would need to open at the end to let the blood out
Atria and ventricles don’t contract simultaneously.
After atria systole, isovolumetric contraction follows. Pressure is being built up in the ventricles, eventually enough to force the AV valves to close (rather abruptly). Only when the built up pressure in ventricles reaches the level that is higher than those in the aorta & pulmonary arteries (~80mmHg) do the semilunar valves open for ventricular ejection.