Which of the Following Is an Example of "Reciprocity" in International

Which of the following is an example of "reciprocity" in international trade?

A) Most of the countries in the European Union have adopted the same currency.
B) France reduced tariffs on wheat imports after Great Britain reduced tariffs on wheat imports.
C) Argentina,Brazil,Uruguay,and Paraguay collectively agreed to a common tariff on external goods.
D) A majority of the world's countries have joined the World Trade Organization.
E) Oil-producing states cooperate on how much oil they will export.