The Organization of the Petroleum Exporting Countries (OPEC) limits its members' oil production based on its "proven reserves." The higher their reserves, the more OPEC allows them to produce. Over the past fifteen years, Saudi Arabia's claimed reserves have been flat, with the exception of an increase of about 100 billion barrels between 1987 and 1988.

According to the passage above, OPEC limits the amount of oil Saudi Arabia can produce. Because this limit on production also limits how much oil Saudi Arabia can export, OPEC's production rule is
A.
an entrepreneur.
B.
a trade barrier.
C.
a currency exchange.
D.
human capital.