Haemoglobin $(Hb)$ transports oxygen from the lungs to tissues. The partial pressure of oxygen in the lungs is different from that in the tissues. Each $Hb$ can bind to up to four oxygen molecules. Suppose we have an equal number of $Hb$ and oxygen molecules,and all the oxygen molecules are in a bound form. Then,which of the following is true?

• A
  Almost all the $Hb$ molecules have one bound oxygen molecule.
• B
  Nearly half of all the $Hb$ molecules are bound to two oxygen molecules.
• C
  Nearly one-fourth of all the $Hb$ molecules are bound to four oxygen molecules each.
• D
  Most of the $Hb$ molecules have one bound oxygen molecule each; the rest either have no bound oxygen or have two or more bound oxygen molecules.