Commentator: The theory of trade retaliation states that countries closed out of any of another country's markets should close some of their own markets to the other country in order to pressure the other country to reopen its markets. If every country acted according to this theory, no country would trade with any other. The commentator's argument relies on which of the following assumptions? ['For any two countries, at least one has some market closed to the other.', 'Countries close their markets to foreigners to protect domestic producers.', 'Trade disputes should be settled by international tribunal.', 'No country actually acts according to the theory of trade retaliation.']