The demand for vaccinations is given by the following: p =200 - Q. The supply of vaccinations is given by: p=50+Q. Further, vaccinations provide an external benefit (b) equal to b=0.5*Q. If a Pigouvian subsidy is used to ensure that the efficient level of vaccinations occur, the value of that subsidy will equal:
a. 0.
b. 20.
c. 30.
d. 50.
e. 100.