Haemoglobin is a Fe containing protein responsible for oxygen transport in the blood. The curves given below indicate the percentage saturation of haemoglobin by $$O_2$$ as a function of partial pressure of $$O_2$$.

Which of the following statement/s is/are correct for the given curves?
I. In presence of $$CO_2$$, higher $$p_{O_2}$$ is needed for a given percentage saturation
II. In presence of $$CO_2$$, lower $$p_{O_2}$$ is needed for a given percentage saturation
III. The maximum percentage saturation is not affected by the presence of $$CO_2$$
IV. In the absence of $$CO_2$$, maximum saturation of haemoglobin occurs at lower $$p_{O_2}$$