30. Read the passage below; then answer the question that follows.
During the mid-1930s, farmers in northern Illinois and eastern Iowa became the first U.S. food producers to adopt hybrid corn. Within a decade, the new seeds were being planted by corn growers throughout much of the Midwest. By mid-century, use of the hybrid varieties had spread to nearly all corn-growing areas of the United States and southern Canada.
The passage above best illustrates the meaning of which of the following geographic concepts?