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?

• ANo country actually acts according to the theory of trade retaliation.  
• BNo country should block any of its markets to foreign trade.  
• CTrade disputes should be settled by international tribunal.  
• DFor any two countries, at least one has some market closed to the other.  
• ECountries close their markets to foreigners to protect domestic producers.