Key Concept: C$_4$ plants have an extra CO$_2$-concentrating step before the Calvin cycle that C$_3$ plants lack.
Why it's correct: Maize uses this C$_4$ pathway to concentrate CO$_2$ at the site of the Calvin cycle, boosting photosynthetic efficiency under high light and heat.
Why others fail: Wheat, rice, and barley are all C$_3$ plants that lack this concentrating mechanism, making them comparatively less water- and nitrogen-efficient in hot conditions.