This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
Einthoven's triangle, what is the value of lead III when lead I = 2 mv and Lead II = 1 mv 
Select and explain. 
A. 1, B. 2, C. 3, D. 4
A: 1
Answer: A: 1 
Explanation: Einthovens formula states that the sum of voltages from lead I plus those from lead III minus those from lead II is zero. Lead I+III-II=0 This is based on the principle that the sum.of the voltages in a closed path equals zero. Because the positive and negative electrodes of lead II are reversed. So it is subtracted instead of adding.