A Typical Hill Plot (Log [Y_O2/(1- Y_O2)] Vs Log P_O2)for

A typical Hill plot (log [Y_O2/(1- Y_O2) ] vs log P_O2) for hemoglobin-oxygen binding does not yield a straight line as it does for myoglobin. Which of the following is the best explanation for this?

A) at high oxygen pressures, hemoglobin is in a low oxygen affinity state
B) at low oxygen pressures, hemoglobin is in a high oxygen affinity state
C) hemoglobin undergoes a transition from low affinity state under low oxygen pressure to a high affinity state under high oxygen pressure
D) hemoglobin undergoes a transition from high affinity state under low oxygen pressure to a low affinity state under high oxygen pressure
E) none of the above