This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, 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: According to Einthoven's triangle,  voltage in I + III = voltage in II
So Voltage in III = II-I
= 1-2 = -1
The value is 1.