$($Last Word$)$ In a cap$-$and$-$trade program,

a$)$ government fixes the price of pollution rights and firms choose how many permits to purchase.

b$)$ government fixes the maximum amount of a pollutant that firms can discharge and issues permits that firms can buy from and sell to each other.

c$)$ each firm is provided a fixed number of permits for a particular pollutant and no individual firm is allowed to acquire additional permits.

d$)$ firms can emit whatever type of pollutant they want, so long as the total tonnage does not exceed a government$-$established quantity.