What is a third degree AV block?
A) a progressive prolongation of the PR interval until an atrial impulse is not conducted by the ventricles
B) a prolonged PR interval representing slowed conduction through the AV node
C) none of the atrial impulses conduct to the ventricles
D) nonconduction of some of the atrial impulses to the ventricle without any change in the PR interval of the conduction impulses