A、They failed to improve their strains of corn. B、They knew very little about cultivating corn. C、Their economies were based on

In the late 1400s, when Christopher Columbus returned to Spain from the western hemisphere, he brought with him a sample of what the native Americans called maize, or as we call it more often today, corn.(23)The corn that Columbus introduced to Europe was the distant descendent of a grass native to Mexico. The peoples of the Americas probably started to domesticate this grass as early as 5000 BC. After about a thousand years, they had developed highly productive strains of corn which later became the basis for the great pre-Columbian civilizations.(24)Figuratively speaking, both the city of the Incas and the temples of the Mayas were built on corn. Domesticated corn and the people who cultivated it developed together. Without humans to care for it, domesticated corn could not survive. The kernels are crowded together beneath the strong, protective husk and silk.(25)And the young corn shoo is not strong enough to break through the husk on its own. If people did not strip away the husk and plant individual kernels, the corn would die out.
23. Where was corn probably first cultivated?
24. What does the speaker imply about the Mayan and Incan civilizations?
25. Why would corn die out without human intervention?