QUESTION $15$

At one time, Pittsburgh, Pennsylvania, was known for its poor air quality. Locals said that a man wearing a white shirt when he walked into the city would have a black shirt when he left. Since that time, the city has greatly reduced its air and water pollution. As a result of cleaner air and water the:

a$.$ standard of living of the citizens of Pittsburgh has increased, even though the improvement in the environment was not reflected in its GDP$.$

b$.$ value of the GDP increased.

c$.$ value of the GDP increased in the year that the improvements were made but did not change in ensuing years.

d$.$ standard of living of the citizens of Pittsburgh did not change.