The correct option is B Difference in electric potential
Electro-cardiograph is a machine used to obtain an electrocardiogram (ECG). ECG is a graphical representation of the electrical activity (difference in electric potential) of the heart during a cardiac cycle. To obtain a standard ECG, a patient is connected to the machine with three electrical leads (one to each wrist and to the left ankle) that continuously monitor the heart activity. For a detailed evaluation of the hearts function, multiple leads are attached to the chest region.