All the components of the conducting system can generate an action potential for the contraction of heart muscle, but the sino-atrial node acts as the pacemaker because :

A

The sino-atrial node has a lower inherent rate of depolarisation

B

All the other components in heart cannot conduct the action potential

C

Only the sino-atrial node is auto-excitable and auto-rhythmic

D

The sino-atrial node has a higher inherent rate of depolarisation