Where Do Economic Agents, Such as Individuals, Firms and Nations

Where do economic agents, such as individuals, firms and nations, interact with each other?

A) In public locations monitored by the government.
B) In any arena that brings together buyers and sellers.
C) In any physical location people where people can physically get together for selling goods, such as shopping malls.
D) In any location where transactions can be monitored by consumer groups and taxed by the government.