Suppose the dollar amount of the externality, per gallon of gasoline, is constant, regardless of how much gasoline is produced. Then the externality could be internalized if producers of gasoline were a. required to pay a tax of $0.45 per gallon of gasoline sold.
b. provided a subsidy of $0.45 per gallon of gasoline sold.
c. required to pay a tax of $0.30 per gallon of gasoline sold.
d. provided a subsidy of $0.30 per gallon of gasoline sold.

a. required to pay a tax of $0.45 per gallon of gasoline sold.

Explanation:

The marginal external cost shows the difference between the private cost and the social cost. Also it should be the tax imposed amount. In the given case, the value is of $0.45 this represent that there is the tax of $0.45 that should be imposed on the producers in order to internalize the external cost

Therefore, the option a is correct

a and c

explanation:

answer; work samples;