博士 是授予的最高学位，因此需要在书面论文中提出的知识和研究成果方面的最高水平和成就。尽管没有特定的课程要求，但大多数博士生大约需要花费四到五年的时间来攻读博士学位，其中包括研究工作以及学士学位后的约50至60个学时。<br> <br> Theworld’根据联合国目前的人口估计，未来几十年该国人口将超过90亿。几乎所有的增长都将来自新兴经济体，在这些新兴经济体中，缺乏现代基础设施会导致各种健康问题–难以获得水或水质差，空气污染，收集和处理废物和垃圾的系统不足。城市为适应不断增长的人口提供了机会-人均
The Ph.D. is the highest degree awarded, and as such requires the highest level of proficiency and achievement, both in knowledge and in the performance of research presented in a written dissertation. While there are no specific course requirements, most doctoral students spend approximately four to five years working toward a Ph.D., which includes research and approximately 50 to 60 credit hours beyond the bachelor's degree. The world's population is projected to exceed 9 billion people in the coming decades, according to current population estimates from the United Nations. Almost all of that growth will come in emerging economies, where the lack of modern infrastructure leads to a variety of health concerns poor access to water or poor water quality, air pollution, inadequate systems for the collection or treatment of wastes and trash. Cities offer opportunities for accommodating this growing population lower per-capita energy consumption, concentrated jobs and social resources, efficiencies in delivering all manner of services that support our civilization. Cities also present their own problems, accounting for roughly three-quarters of all energy use and carbon emissions on the planet and placing a significant burden on the environment.