Gas exchange between maternal and foetal blood takes place in the placenta. Foetal and adult haemoglobin differ slightly in composition, and their dissociation curves are different so that oxygen can be transferred from maternal blood to foetal blood. If curve III represents the dissociation curve of adult haemoglobin, which curve would represent foetal haemoglobin? (Hint: Must foetal haemoglobin have a higher or lower affinity for oxygen than maternal haemoglobin has?)

• A.  I                  
• B.                    II
• C.  III                                           
• D. IV