Which of the following helped reduce sulfur dioxide emissions ?

(i)	corrective taxes
(ii)	tradable pollution permits
(iii)	amendments to the Clean Air Act

a.	(i) only
b.	both (i) and (ii)
c.	(iii) only
d.	both (ii) and (iii)

the answer is
D , why?what about (i)? I think (i) is reasonable as well?


Sorry that the Chinese typing is not available for me at present :)