An ECG technician is placing leads on a patient who has presented to the emergency department with a sudden onset of chest pain. The technician would recognize which of the following facts about the placement of leads and the achievement of a clinically accurate ECG?

a) A total of 12 chest leads is necessary to attain the most accurate ECG.
b) Limb leads measure the electrical activity of the heart indirectly through the activity of adjacent skeletal muscle.
c) The chest leads measure electrical activity on the horizontal plane, while limb leads measure it on the vertical plane.
d) The electrical potential recorded by a lead on an extremity will vary significantly depending on where the lead is placed on the extremity.