In the Context of a Country's International Trade, a "Gravity

In the context of a country's international trade, a "gravity model" is usually employed to investigate, for the country,

A) the determinants of the amount or volume of the country's trade with its trading Partners.
B) the determinants of the country's terms of trade with its trading partners.
C) the value of the country's "Leontief statistic."
D) whether the Stolper-Samuelson theorem is valid for the country.