Why has there been an increase in the inequality of income?

a. The demand for skilled workers has declined.
b. The demand for skilled workers has remained constant and the demand for unskilled workers has declined.
c. The government policies went against the unskilled workers.
d. The demand for skilled workers has increased and the demand for unskilled workers has not kept pace.
e. The demand for unskilled workers has increased as they could be hired for lower wages.

d