a) The Heckscher-Ohlin theory, also known as the factor proportions theory, suggests that countries will specialize in and export goods that require abundant factors of production, such as labor, capital, or land, that they possess in abundance. Conversely, countries will import goods that require factors of production that are scarce domestically. This theory assumes that factors of production cannot move freely between countries and that there are differences in factor endowments across countries. b) The Stolper-Samuelson theorem states that in a two-country model, when there is an increase in the price of a good, the factor of production used intensively in the production of that good will benefit, while the other factor will lose. For example, if the price of a labor-intensive good increases, then labor will benefit while capital will lose. This theorem predicts that changes in relative prices will affect the distribution of income between factors of production. c) The COVID-19 pandemic has had a significant impact on global trade flows. The restrictions on movement, lockdowns, and disruptions in supply chains have led to a decline in international trade. Many countries have imposed export restrictions on essential goods, leading to a decrease in trade volume. Additionally, the shift