In the estimation of international trade's casual effect on country's income, InYa+BT+yWi+i, = where Y; denotes income per person, T; denotes international trade, W; denotes within-country trade and ; denotes the error term. Please try to explain the possible endogenetiy of international trade and why the proximity, i.e., the distance between a given country and every other country in the world, could be used as a good instrumental variable for international trade.