In the first paragraph of text 18A3-I, the expression which best su...

Text 18A3-I 

    The roar of a waterfall suggests the power of water. Rampaging floodwaters can uproot strong trees and twist railroad tracks. When the power of water is harnessed, however, it can do useful work for humans. 

     Since ancient times, people have put the energy in the flow of water to work. They first made water work for them with the waterwheel, a wheel with paddles around its rim. Flowing water rotated the waterwheel, which in turn ran machinery that was linked to it. Today, new kinds of waterwheels – turbines – spin generators that produce electricity. Electricity from waterturned generators is known as hydroelectricity. 

     By building a dam across a river, the natural upstream water level is elevated and a difference in head is created that can be used to drive turbines and generate electricity. A large upstream reservoir may balance seasonal water flow; rain or melted snow can be stored in the reservoir during the wet season to provide electricity during dry seasons. 

    Waterpower is distributed unevenly among the continents and nations of the world. Europe and North America have developed much of their waterpower. Asia, South America, and Africa have abundant waterpower potential, but while countries such as China and Brazil have become leading hydroelectric producers, much of the waterpower resource on those continents remains undeveloped. 

Elizabeth Lachner. Hydroelectricity. Rosen Publishing Group, 2018 (adapted).