Which of the Following Explains the Conversion of Hemoglobin Subunits $\alpha$

Which of the following explains the conversion of hemoglobin subunits from deoxy to oxy state?

A) all four subunits switch simultaneously
B) each subunit switches only upon oxygen binding to the particular subunit
C) both $\alpha$ subunits simultaneously switch first followed by both $\beta$ subunits
D) both $\beta$ subunits simultaneously switch first followed by both $\alpha$
subunits
E) none of the above