Atrial and ventricular systole do not overlap under normal condition because of which of the following? a) a delay of the action potential at the atrioventricular node
b) rapid conduction by the internodal pathway
c) rapid conduction by the bundle branches
d) the atriventricular node setting the pace of contractions

) a delay of the action potential at the atrioventricular node

Explanation:

The reason  for the short delay of 0.9s. is to ensure maximum contractions of the atrial to eject blood into the relaxed ventricles.(ventricular filing).

Thus when the atrial is contracting,the ventricles relax and vice versa.The short duration for the  artrial contraction to occur for blood ejection into the relax ventricles guarantee that artial systole can not overlap the ventricle systole.

The artial must contract first for the ventricle fillings,and when this is completed,the atrioventricular valves closes,and blood flow through the aorta to the systemic circulation from the left ventricle and into the pulmonary circulation through the pulmonary vein.

This occurs  simultaneously with both the R and L Artrial,and right and left Ventricles.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."