Red blood cells can contain a molecule called 2,3-bisphosphoglycerate (2,3BPG). When 2,3BPG binds to haemoglobin, a greater partial pressure of oxygen is required to cause $50\%$ saturation of haemoglobin with oxygen. Which of these statements about the effect of 2,3BPG are correct?
- A1 and 3
- B2 and 4
- C1 only
- D2 only