Use marginal analysis to answer this question. the optimal amount of pollution reduction is where, from a social perspective:

the marginal benefit of pollution reduction is maximized.
total costs equals total benefits.
marginal costs equals marginal benefits.
marginal benefit equals zero.
which of the following likely imposes the smallest negative externality?

your roommate begins practicing the drums at night while you are asleep.
your roommate goes on a diet
your roommate begins smoking (and you do not smoke)
your neighbor begins collecting junk cars on her front lawn and working on them at night.