The graph illustrates how varying partial pressures of carbon dioxide (CO$_2$) affect the dissociation curve of haemoglobin. What is the difference in percentage oxygen saturation of haemoglobin at a partial pressure of oxygen of $6\,\text{kPa}$ when the partial pressure of carbon dioxide changes from $1.0\,\text{kPa}$ to $1.5\,\text{kPa}$?
- A$-26\%$
- B$-20\%$
- C$20\%$
- D$46\%$