What physical process could geographers track with an arrow on flow-line thematic map?

AA flow-line thematic map is a map that emphasizes a particular theme. They do not show general geographic features.  Geographers are able to track the following events with arrows on a flow-line thematic map: The directions of sea currents, wind directions within a hurricane or similar weather system, shipping routes from one country to the next country, flight patterns of aeroplanes that travel from one country to another country, the number of people that have migrated from one country to another country, as well as many more similar events. 

My answer is letter d

Answer herbicide-resistant soybeans;