The concerns about world food production raised by Malthus have not materialized because:

A) input prices have fallen over time.
B) crop prices have risen over time.
C) Malthus was wrong about the diminishing returns to labor in agriculture.
D) technological improvements have increased our ability to produce food over time.

Technological improvements have greatly increased our ability to produce food over time, allowing for more efficient production, increased crop yields, and the ability to grow crops in areas previously unsuitable for agriculture. This has alleviated concerns about world food production and has allowed for a growing global population to be fed adequately. Input prices and crop prices may fluctuate but overall, the advancements in technology have allowed for the food supply to keep up with demand. Malthus' predictions did not account for the potential of technological advancements.