The concept of the of distance is illustrated by the fact that as cost increases with distance, interactions between people decrease.
a. flexibility
b. friction
c. erosion
d. relativity

Answer;

Friction

Explanation;The concept of the friction of distance is illustrated by the fact that as cost increases with distance, interactions between people decrease.The concept of friction of distance is based on the idea that distance  requires some amount of effort,time, energy, and/or other resources to overcome.Due to friction of distance spatial interactions such as human migration and transport will appear to take place often over shorter distances.