Mexico City's most pressing environmental issue is A) mountain erosion. B) dangerous air pollution. C) erosion along the coastline. D) cutting down of the rainforest.

In 1992, the United Nations described Mexico City’s air as the most polluted on the planet. Industrial growth, a population boom (from three million in 1950 to some 20 million today), and the proliferation of vehicles are the factors that contributed to the huge air pollution.