What Lesson Was Learned from the Imposition of the Smoot-Hawley

What lesson was learned from the imposition of the Smoot-Hawley tariff?

A) Nations can usually work out their differences without any structure or international organization.
B) Tariffs are often necessary to keep trade fair.
C) Retaliation and tariff escalation are not as harmful as many suppose.
D) A reduction of trade as a result of a tariff imposes large costs on the countries involved.