If the ivory trade continues, experts believe, the elephant will soon become extinct in Africa, because poaching is rife in many areas. A total ban on ivory trading would probably prevent the extinction. However, the country of Zimbabwe -- which has virtually eliminated poaching within its borders and which relies on income from carefully culling elephant herds that threaten to become too big -- objects to such a ban. Zimbabwe holds that the problem lies not with the ivory trade but with the conservation policies of other countries. Which one of the following principles forms a logical basis for Zimbabwe's objection to a ban? ['Effective conservation cannot be achieved without eliminating poaching.', 'International measures to correct a problem should not adversely affect countries that are not responsible for the problem.', 'Freedom of trade is not a right but a consequence of agreements among nations.', 'Prohibitions affecting several countries should be enforced by a supranational agency.']