According to paragraph 4, which of the following was true about ancient Mayan communities? A. They each created their own separate trading networks with communities in the nearby highlands. B. They all had many luxury materials that they were able to trade for resources that they lacked. C. They all needed to obtain a number of important materials through trade with other regions. D. They all gradually reduced their trading activities with communities in the Valley of Mexico and developed trading networks with other regions.