meanings of some phrases The scope of the problem facing the world's cities is immense. In 1992, the United Nations Environmental Programmed and the World Health Organization (WHO) concluded that allof a sample of twenty megacities likely to have more than ten million inhabitants in the year 2000 - already exceeded the level the WHO deems healthy in or least one major pollutant. Two-thirds of them exceeded the guidelines for two, seven for three or more. What does "all of a sample" mean here? I‘m really really confused with all those figures in the last sentence.What does"two","seven""three"refer to? Thank you!typing mistake:the fourth line-in at least one major pollutant