Haemoglobin is a Fe containing protein responsible for oxygen...

Haemoglobin is a Fe containing protein responsible for oxygen transport in the blood.. The curves given below indicate the percentage saturation of haemoglobin by $\mathrm{O}_2$ as a function of partial pressure of $\mathrm{O}_2$.
Which of the following statement/s is/are correct for the given curves? I. In presence of $\mathrm{CO}_2$, higher $\mathrm{p}_{\mathrm{O} 2}$ is needed for a given percentage saturation II. In presence of $\mathrm{CO}_2$, lower $\mathrm{p}_{\mathrm{O} 2}$ is needed for a given percentage saturation III. The maximum percentage saturation is not affected by the presence of $\mathrm{CO}_2$ IV. In the absence of $\mathrm{CO}_2$, maximum saturation of haemoglobin occurs at lower p O 2