translation.en: We have a challenge, because the urban population is growing in this century from two billion to six billion in a very short amount of time. People are moving to the cities.
translation.zh: 然而我们有个严峻的挑战，因为在本世纪 短短的一段时间内，城市人口已经从 20亿增加到了60亿。人们在往城市迁徙。