听力原文： Today, most countries in the world have canals. Even in the 20th century, goods can be moved more cheaply by boat that any other means of transport. Some canals such as the Suez or the Panama, save ships weeks of time by making their voyage a thousand miles shorter. Other canals permit boats to reach cities that are not located on the coast. Still other canals drain lands where there is too much water, help to irrigate fields where there is not enough water. In places where it does not rain very often, irrigation canals drain water from rivers or lakes such as the lake behind the Aswan Dam on the Nile River, and provide the irrigation water. In places where there is too much water, canals can drain the water off the land for use in fanning. In Holland, acres and acres of land have been drained in this way. Since much of this drained land is below sea level, the water in the canals has to be pumped up to sea level. Dikes have been built in Holland to keep the sea from covering the land, as it did in the past. Canals are also used to carry water to mills and factories. The water from a river is kept at a higher level than the river until it reaches the wheel of the mill. Then the water is poured over un61 it reaches the wheel of the mill. Then the water is poured over the mill wheel, making it turn. The same principle is used in more modern factories and in hydroelectric generating plants. The force of the water, falling from a certain height, provides a cheap way of producing electricity. (30)