5. Which of the following is true? a. A customs union imposes a common external tariff on goods imported into the customs union while maintain tariffs on the exchange of goods between participating countries. b. a customs union imposes a common external tariff on goods imported into the customs union and eliminates tariffs on the exchange of goods between participating countries. c. a customs union imposes common policies for the internal operation and integration of the combined market between the participating countries. d. both (b) and (c) are true