Which of the following is true? a. Countries with higher trade barriers have higher growth rates. b. The size of the trade sector (exports plus imports) as a share of GDP will generally be larger in more populous countries than in smaller less-populated countries. c. When economies of scale are important in an industry, the domestic market of a small country may not be large enough to support cost-efficient firms. d. In small countries, firms in industries where economies of scale are important will tend to export little, if any, of their output