Which description correctly identifies a mature red blood cell?

• A. No nucleus; no protein production
• B. Carry O2 and Dioxides
• C. Live for 120 days
• D. Bi-concave discs (high carrying capacity, flexible for small spaces)

Answer: (D)

The key idea is how the shape of a mature red blood cell supports its job. A mature red blood cell is a bi-concave disc, which gives it a high surface-area-to-volume ratio and makes it very flexible. That combination lets oxygen and carbon dioxide diffuse efficiently across the cell membrane and allows the cell to squeeze through narrow capillaries in the bloodstream. While red blood cells do transport oxygen and carbon dioxide and in many species have a lifespan around 120 days, these facts describe function or duration rather than the defining appearance. The bi-concave form is the most distinctive feature that identifies a mature red blood cell, explaining why this description is the best choice.