The following graph shows the market for pollution rights.Which of the following couldnothave caused an increase in the demand for pollution rights?

a. A decrease in the cost of pollution abatement equipment

b. The subsidy provided by the government on the price of pollution rights

c. An increase in the number of polluting firms

d. The ability of existing firms to sell pollution rights at a price lower than the market price

e. An increase in the ability of the firm to absorb pollutants