(a) Show that if all individuals in a country have the same holdings of all factors of production (and the same shareholdings in all companies), and if there are no externalities, the opportunity to trade must lead to an unambiguous Pareto improvement in that country. In this sense, lobbying for trade barriers must come from the uneven distribution of factor endowments across people.

(b) Provide an example in which there are externalities and the opening-up to trade makes all individuals worse off, even in this completely symmetric world.