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
2nd Most Common Hb in adult is 
Select and explain. 
A. a 2 ss 2, B. a 2 g 2, C. a 2 d 2, D. ss 2 g 2
C: a 2 d 2
Answer: C: a 2 d 2 
Explanation: a 2 ss 2 is the most abundant Hb in adult About 2.5% of the hemoglobin is hemoglobin A2, in which b chains are replaced by d chains (a2d2) which is second most abundant Hb in adult humanRef: Ganong's Review of Medical Physiology Twenty-Third Edition Page No: 522