According to the gravity model
O
A the amount of interaction between two cities is based on the amount of resources available in those places.
B the amount of interaction between two cities is based on the distance between them.
C the amount of interaction between two cities is based on the type of work done in each.
D the amount of interaction between two cities is based on the size of the cities and the distance between them.
E the amount of interaction between two cities is based on the available transportation.