Natural rate of unemployment can be viewed as comprising frictional unemployment and structural unemployment. Which one of the followings is not related to frictional unemployment?

a. Frictional unemployment is caused by the time it takes to match workers and jobs given different preferences and skills of workers and job's requirements.

b. Sectoral shifts (rising vs. declining industries)

c. Minimum wage law

d. firm dynamics such as entries of new firms (that hire new workers) and failures of the existing firms (that have to let go workers)

e. Public policy such as unemployment insurance and job training programs.