Final answer:
The Gravity Model equation in the context of social sciences primarily relies on Population and Distance as its variables, analogous to mass and the inverse-square of distance in Newton's law of gravitation.
Step-by-step explanation:
The two important variables of the Gravity Model equation are Population and Distance. This is option D. The Gravity Model, often used in social sciences to predict and describe certain behaviors, such as trade flows or migration patterns, relies on these two variables to quantify the potential interaction between two places. The population represents the mass component of the gravitational attraction, as larger populations exert a greater 'pull' similar to what mass does in physics. Meanwhile, the distance factor aligns with the inverse-square law from Newton's law of gravitation, where the force between two bodies decreases with the square of the distance between them.