What structure is the only place where an action potential can cross to ventricles?

A) sinoatrial node
B) atrioventricular node
C) gap junction
D) the left atrium
E) conducting fiber

The atrioventricular node (AV node) is the only place where an action potential can cross from the atria to the ventricles. This is because the AV node is the only electrical connection between the two chambers, and it acts as a gatekeeper, controlling the flow of electrical impulses from the atria to the ventricles. The sinoatrial node (SA node), on the other hand, is responsible for initiating the heartbeat, but it does not necessarily regulate the conduction of the impulses. Gap junctions and conducting fibers are not specific structures that regulate the flow of electrical impulses between the atria and ventricles, and the left atrium is not directly involved in this process.