Identify whether each description describes lead I, lead II$,$ or lead III:

a$.$ measures the difference in electrical potential between the right arm $($RA$)$ and the left leg $($LL$).$

b$.$ measures the difference in electrical potential between the right arm $($RA$)$ and the left arm $($LA$).$

c$.$ measures the difference in electrical potential between the left arm $($LA$)$ and the left leg $($LL$).$