Blockage Is Ideally Accomplished

Blockage is ideally accomplished:

A) when a country refuses to allow an importer to exchange its national currency for the seller's currency.
B) when two countries enter into a voluntary agreement to restrict volume of exports.
C) when a country applies specific unit or dollar limit to a particular type of good.
D) when countries limit the export of certain goods on a case-by-case basis.
E) when the government imposes a mandatory tax on goods entering at its borders.