Which of the following correctly compares the effects of BPG, carbon dioxide, and protons on hemoglobin's oxygen binding curve?

• A. All three shift the curve to the right, decreasing oxygen affinity.
• B. Protons shift the curve to the left, while BPG and carbon dioxide shift it to the right.
• C. Carbon dioxide shifts the curve to the left, while BPG and protons shift it to the right.
• D. BPG shifts the curve to the left, while carbon dioxide and protons shift it to the right.

Answer: (A)