Which of the following statements correctly defines the $Bohr$ effect?

• A
  Rise in $P_{CO_2}$ with a decrease in $CO_2$ concentration.
• B
  Rise in $P_{CO_2}$ with an increase in $CO_2$ concentration.
• C
  Rise in $P_{CO_2}$ with an increase in $CO_2$ and decrease in $P_{O_2}$.
• D
  Shift of the oxygen-haemoglobin dissociation curve to the right due to increased $CO_2$ concentration or decreased $pH$.