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  3. Canals are watercourses constructed to improve and extend natural waterways

Canals are watercourses constructed to improve and extend natural waterways

游客2023-11-23  50
问题      Canals are watercourses constructed to improve and extend natural waterways. They are generally built to facilitate transportation, but from the beginning they have been used for many additional purposes including draining swamps, irrigating land for cultivation, and promoting economic development.
     Canals are often classified by the size of vessel they can accommodate. Some small local canals, which are able to float only 100 to 300 ton boats or small rafts of timber, may be only 3 feet deep. Major barge canals generally range from 6 to 9 feet in depth, and some are as much as 10 or 12 feet deep. These canals can carry 1,350 to 2,000 ton craft. Ship canals are 25 feet or more deep and are capable of accommodating large vessels in the seagoing class.
     Canals may also be classified as either water level or lock canals. Water level canals do not vary in height along their courses. The best known of these is the Suez Canal, which is at sea level. Lock canals, which include most modem waterways, contain locks, or special devices for raising and lowering boats along their courses by changing the depth of the water. Each lock is a stretch of water enclosed by gates at each end. After a boat enters the lock, water is let or drained out until it reaches approximately the same level as the water ahead. [br] What does the passage mainly discuss?
选项 A、How canals are constructed.
B、Common types of canal boats and barges.
C、The world’ s largest canals.
D、How canals are used and classified.
答案 D
解析 通读全文不难发现，每段第一句即为段落主题句，第一段讲什么是canal，第二、三段讲的canal不同分类及其作用，因而本题应该选D。
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