Which of the following is true?

a. 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.
b. In small countries, firms in industries where economies of scale are important will tend to export little, if any, of their output.
c. 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.
d. Countries with higher trade barriers have higher growth rates.

A