Why does BPG binding in erythrocytes selectively affect hemoglobin's oxygen activity in tissues but not in the lungs?

• A. BPG binds to the t state in tissues, which is not present in the r state in the lungs.
• B. BPG does not bind in erythrocytes, affecting oxygen activity only in the lungs.
• C. BPG binds equally in both tissues and lungs, affecting oxygen activity similarly.
• D. BPG binds to the r state in tissues, which is not present in the t state in the lungs.

Answer: (A)