Smoking leads to ................. in the blood.

• A
  Increase in $O_2$ and decrease in $CO_2$ bound to $Hb$
• B
  Decrease in $O_2$ and increase in $O_2$ bound to $Hb$
• C
  Increase in $CO$ and decrease in $O_2$ bound to $Hb$
• D
  Decrease in $CO$ and decrease in $CO_2$ bound to $Hb$