托福阅读official65 Which of the sentences below best expresses the essential information in the highlighted sentence in答案解析【小站备考】

Geographical constraints have had important consequences for the economy and society of Iran. Where rainfall is adequate, there are fertile valleys and grasslands suitable for grazing animals. However, since the natural vegetation tends to be sparse, it is difficult for such animals to remain in one place for any length of time. Thus nomadic pastoralism—keeping livestock (such as sheep and goats) by wandering from place to place—was one of the first and most persistent human economic activities to flourish in this area. This nomadic movement was often of the vertical variety, with people and animals moving from lowlands in wintertime to highlands in summer. The animals raised by the pastoralists provided not only food but also material for crafts such as the making of carpets, thick felt cloth, and tents. The pastoralists were typically organized into large tribal confederations capable of controlling the vast territories needed for maintaining their herds.

The tribes were a powerful social and political factor throughout Iranian history. The skills necessary for herding animals, hunting and chasing off predators, directing migrations, disciplining tribesmen, and protecting lands and animals from rivals could be easily adapted and directed toward military purposes as well. It was typically the tribes that produced the soldiers and rulers of the country and provided the power base for most of its dynasties. Once established, governments needed to cultivate the support of friendly tribal groups and tried to control hostile tribes by combat, deportation, or forcible settlement. At the beginning of the twentieth century, approximately one-fourth of the population were tribal peoples, and they were a potent force in Iranian affairs. With the advent of mechanized armies in the 1930s, however, there were systematic efforts to break the power of the tribes and to coerce the tribal population into a sedentary way of life. These efforts have been largely successful, and the tribes are no longer so significant a force in either the Iranian economy or society. Less than 5 percent of the population now consists of nomadic pastoralists.

The aridity of the Iranian plateau retarded its agricultural development in comparison to adjacent regions such as Mesopotamia, which had great rivers to draw upon for a supply of water. Eventually, at some uncertain date probably about 26 centuries ago, there was a technological breakthrough that made it possible to farm crops outside the few oases, streams, and other places with sufficient rainfall for agriculture. This was the development of underground canals known as qanats. The qanat system took advantage of the natural slope (inclination) of the plateau basins. A well would be dug in the foothills to reach a water source, usually water from melting snow that had seeped underground. Then a sequence of wells and shafts connected by underground canals would be constructed to transport the water to an area suitable for cultivation, where it could support the needs of one or more villages. The slope of the underground canals had to be controlled carefully to prevent erosion, and the interior surface of wells and shafts needed to be kept under constant maintenance to prevent them from collapsing.

Since the canals were underground, loss from evaporation was minimized. Gravity provided the means of moving the water, so no mechanical energy was required to operate the system. The numerous wells and shafts kept the length of the tricky underground canal short and facilitated repairs of each segment. Built up over the centuries, the system eventually became immense. It has been estimated that the total length of the qanat system today, counting wells, shafts, and canals, is in excess of 300,000 kilometers almost the distance from Earth to the Moon!, which gives some idea of the tremendous investment in money and labor power it represents. Yet the type of agriculture that developed around the qanat system gave modest yields and required hard work from the peasant farmers, who received only a small share of the agricultural produce.

OFFICIAL65 Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

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