Concept:Natural resources exist naturally, not because of human effort.
Explanation:Natural resources are provided by nature, such as air, water, forests, and minerals.
They are not made by humans, so they are often described as free gifts of nature.
Option A is incorrect because natural resources can be limited and exhaustible.
Option B is incorrect because their cost of production is not a defining feature.
Option D is incorrect because they can command a price once processed or made scarce.
Therefore, the best description is that they are free gifts of nature.
Answer:C. are free gifts of nature