How do BPG, carbon dioxide, and protons affect hemoglobin's oxygen binding curve?

• A. They all shift the curve to the right, decreasing hemoglobin's oxygen affinity.
• B. BPG has no effect, while carbon dioxide and protons shift the curve to the right.
• C. BPG shifts the curve to the right, while carbon dioxide and protons shift it to the left.
• D. They all shift the curve to the left, increasing hemoglobin's oxygen affinity.

Answer: (A)