Which factor is named as influencing the natural rate of unemployment?

1. The natural rate is not a single fixed number. 2. The natural rate is the unemployment that persists in a healthy economy. 3. Another factor that influences the natural rate is named directly. 4. That factor is the amount of structural unemployment. _Source: OpenStax Principles of Macroeconomics for AP Courses 2e (CC BY 4.0), Ch 7 'Unemployment'_