Which of the Following Two Elements Are Both Central to "Gravity

Which of the following two elements are both central to "gravity" models of international trade?

A) Countries- latitudinal and longitudinal locations along the surface of the planet.
B) Countries- sizes and geographic distances separating them.
C) Depths of waterways and heights of surfaces along which international trade flows occur.
D) The relative weights that nations- residents place on physical goods versus intangible services when voting on trade policies.