In a natural experiment, the same cross-sectional units appear in each time period.

In general, in the 19th century, America

(a) was a low tariff nation because it was believed that free trade brought specialization, efficiency and more rapid economic growth. (b) was a nation that kept its tariffs at about the same levels as England so as not to give the British an advantage. (c) was a high tariff nation which believed, from the days of Alexander Hamilton, that America's industry needed protection from the more industrially advanced England. (d) had a moderate level of tariffs, compared with England, whose main purpose was to provide the federal government with revenues.

Going to college to get more knowledge and skills is considered to be an increase in

A) physical capital. B) consumption. C) human capital. D) net exports.