This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: ECG, Subject name: Physiology
When recording lead I on an ECG, the right arm is the negative electrode and the positive electrode is the 
Select and explain. 
A. Left arm, B. Left leg, C. Right leg, D. Right arm + left leg
A: Left arm
Answer: A: Left arm 
Explanation: By convention, the left arm is the positive electrode for lead I of an ECG.