You are in charge of tariff policy for your country. You are asked to place an import restraint on products from a politically-allied nation. The restraint must be subtle and most difficult to detect and be "safe" from attack in the press. Your best suggestion is to implement: a. A global tariff with all nations treated equally. b. Use of indirect non-tariff barriers by administrative regulations which are facially neutral but which impact imports by ally negatively as applied. c. A complex licensing scheme for imports from ally. d. Use of an ad valorem tariff on goods from ally