A population is living off a secure subsistence base, eating plants and animals caught by hunting, fishing, and gathering. The population expands, and as it does, it puts pressure on the resource base, forcing people to eat "third-choice" foods, especially grains. They discovered that the grain responded to human efforts to increase yields, and came to rely increasingly on it. This scenario would fit best with which of the following theories of agricultural development?
A. Broad spectrum foraging theory.
B. Conflict theory.
C. Resource base transformation theory.
D. Population pressure theory.