The natural rate of unemployment is A. the sum of structural unemployment and cyclical unemployment. B. the sum of cyclical unemployment and frictional unemployment. C. the sum of seasonal unemployment and cyclical unemployment. D. the sum of structural unemployment and frictional unemployment. When the economy is at full​ employment, unemployment is equal to A. normal rate of unemployment minus abnormal rate of unemployment. B. cyclical unemployment minus seasonal unemployment. C. zero. D. the natural rate of unemployment.