The Most Basic Cause of Unemployment in the United States

The most basic cause of unemployment in the United States is that

A) the natural rate of unemployment is much higher than zero.
B) we are most of the time producing a real GDP below the natural real GDP.
C) we are suffering from low productivity growth.
D) we are converting from a predominantly industrial to a predominantly service-based economy.
E) policymakers are constantly trying to hold down inflation.