Photosynthesis in ${\mathrm{C}}_4$ plants is relatively less limited by atmospheric ${\mathrm{CO}}_2$ levels because
1. The primary fixation of ${\mathrm{CO}}_2$ is mediated via PEP carboxylase.
2. Effective pumping of ${\mathrm{CO}}_2$ into bundle sheath cells.
3. Four carbon acids are the primary initial ${\mathrm{CO}}_2$ fixation products.
4. RuBisCO in ${\mathrm{C}}_4$ plants has higher affinity for ${\mathrm{CO}}_{2.}$

Consider the two statements:

I. The C₄ photosynthesis pathway uses more energy than the C₃ photosynthesis pathway.
II. C₄ plants are able to more efficiently fix carbon in drought, high temperatures, and in limitations of nitrogen or CO₂.

1. Both I and II are correct and II explains I
2. Both I and II are correct but II does not explain I
3. Only I is correct
4. Both I and II are incorrect