Final answer:
The electrical impulses generated by the pacemaker cells at the sinoatrial (SA) node travel to the atria and ventricles, causing them to contract together and ensuring efficient pumping of blood throughout the body.
Step-by-step explanation:
The most reasonable explanation for nearly simultaneous contractions of the Left and Right atria, when the primary pacemaker is localized to the R atrium, is that the electrical impulses generated by the pacemaker cells at the sinoatrial (SA) node rapidly travel first to the right and left atria, causing them to contract together. These electrical signals then travel to the atrioventricular (AV) node and from there to the right and left ventricles, which also contract together. This coordinated contraction of the atria and ventricles ensures efficient pumping of blood throughout the body.