Wage Inequality Has Been on the Rise in Virtually All

Wage inequality has been on the rise in virtually all high-income industrial economies since the 1970s.The causes are probably numerous,but the leading explanation for the greatest share of the increase in inequality is

A) the growth of trade with developing countries.
B) the growth of trade with other high income industrial countries.
C) technological change which increased the relative demand for skilled workers.
D) technological change which increased the relative demand for unskilled workers.