Which of the following organizations can be considered to promote a barrier to entry to its industry?

Question $10$Select one:

a$.$

Iowa's Manure Applicator Certification Program $($seriously: "State law requires most manure applicators in Iowa to be certified."$)$

b$.$

All of these organizations promote barriers to entry $($knowingly or unknowingly$).$

c$.$

The American Medical Association $($which certifies physicians$).$

d$.$

Teachers' unions $($which certifies teachers$).$