This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
S.A. node acts as a pacemaker of the hea because of the fact that it 
Select and explain. 
A. Is capable of generating impulses spontaneously, B. Has rich sympathetic innervations, C. Has poor cholinergic innervations, D. Generates impulses at the highest rate
D: Generates impulses at the highest rate
Answer: D: Generates impulses at the highest rate 
Explanation: Some cardiac fibers have the capability of self-excitation, a process that can cause automatic rhythmical discharge and contraction. This capability is especially true of the fibers of the hea 's specialized conducting system, including the fibers of the sinus node. For this reason, the sinus node ordinarily controls the rate of beat of the entire heaRef: Guyton and Hall textbook of medical physiology 13th edition. Page: 123