This following text is for questions 77 to 78.
 

    Among the environmental specters confronting humanity in the 21st century - global warming, the destruction of rain forests, overfishing of the oceans - a shortage of fresh water is at the top of the list, particularly in the developing world. Hardly a month passes without a new study making another alarming prediction, further deepening concern over what a World Bank expert calls the "grim arithmetic of water." Recently the United Nations said that 2.7 billion people would face severe water shortages by 2025 if consumption continuous at current rates. Fears about a parched future arise from a projected growth of world population from more than six billion today to an estimated nine billion in 2050. Yet the amount of fresh water on Earth is not increasing. Nearly 97 percent of the planet's water is salt water in seas and oceans. Close to 2 percents of Earth's water is frozen in polar ice sheets and glaciers, and a fraction of one percent is available for drinking, irrigation, and industrial use.

    Gloomy water news, however, is not just a thing of the future: Today a n estimated 1.2 billion people drink unclean water, and about 2.5 billion lack proper toilets or sewerage systems. More than five million people die each year from water-related diseases such as cholera and dysentery. All over the globe fa rmers and municipalities are pumping water out of the ground faster than it can be replenished.

    Still, as I discovered on a two-month trip to Africa, India, and Spain, a host of individuals, organizations, and businesses are working to solve water's dismal arithmetic.

The subject matter discussed in the passage is ....