Classify the following as a government-enforced barrier to entry, a barrier to entry that is not government-enforced, or a situation that does not involve a barrier to entry.

a. A city passes a law on how many licenses it will issue for taxicabs

b. A city passes a law that all taxicab drivers must pass a driving safety test and have insurance

c. A well-known trademark

d. Owning a spring that offers very pure water

e. An industry where economies of scale are very large compared to the size of demand in the market