20. Explain how central place theory is used to describe the spatial distribution of cities in a region. 21. Define threshold. 22. Define range.

20. Central place theory is a geographical theory that explains the spatial distribution of cities and towns in a region. It was developed by the German geographer Walter Christaller in the 1930s. According to this theory, cities and towns are strategically located to serve the surrounding areas, providing goods and services to the people living in those areas. The theory suggests that cities and towns are arranged in a hierarchical pattern, with larger cities serving a larger population and smaller towns serving a smaller population. The theory also suggests that the location of cities and towns is determined by factors such as transportation, communication, and the availability of resources.21. In geography, a threshold refers to the minimum number of people or activities required to support a particular service or function. For example, a town may have a threshold of 10,000 people to support a shopping mall, or a certain number of residents to support a school or hospital. The threshold is the minimum number of people or activities required to make a particular service or function viable.22. In geography, a range refers to the maximum distance people are willing to travel to access a particular service or function. For example, the range for a shopping mall may be 20 kilometers, meaning people are willing to travel up to 20 kilometers to shop at the mall. The range for a hospital may be 50 kilometers, meaning people are willing to travel up to 50 kilometers to access the hospital's services. The range is the maximum distance people are willing to travel to access a particular service or function.