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
Biconcave shape of RBC helps by 
Select and explain. 
A. Increasing flexibility, B. Increasing surface area, C. Carrying more Haemoglobin, D. Passing easily through smaller capillaries
D: Passing easily through smaller capillaries
Answer: D: Passing easily through smaller capillaries 
Explanation: The shapes of RBCs can change remarkably as the cells squeeze through capillaries. Actually, the RBC is a "bag" that can be deformed into almostany shape.