Three firms put 4 tons of noxious gases into the atmosphere each in a year. The total abatement costs for each firm are
$$
\begin{aligned}
& T A C_1=5 e_1^2, \\
& T A C_2=10 e_2+5 e_2^2, \\
& T A C_3=20 e_3+5 e_3^2,
\end{aligned}
$$
where $e_i$ is the amount of the gas emitted by firm $i$.
(a) Suppose the government wants to cut the total emissions from 12 units to 6 units by direct regulation, requiring each firm to cut back on two units of emission each. How much will this plan cost overall?
(b) Suppose the government wants to cut the total emissions from 12 units to 6 units. What is the socially efficient (i.e., that costs the least to society) way to cut back on emissions? How much will each firm cut back in this case?
(c) Suppose the government wants to cut the total emissions from 12 units to 6 units using an emissions tax. What level of tax will bring this about? How much will the tax revenue collected be?