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  3. Canal Today, most countries in the world have canals. Even in the twentieth century, goods can be moved more cheaply by boat(51)

Canal Today, most countries in the world have canals. Even in the twentieth century, goods can be moved more cheaply by boat(51)

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问题 Canal
Today, most countries in the world have canals. Even in the twentieth century, goods can be moved more cheaply by boat(51) by any other means of trans- port. Some canals, such as the Suez or the Panama,(52) 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(53) lands where there is too much water, help or irrigate fields where there is not enough water, and furnish water power for factories and mills.
The(54) of a canal depends on the kind of boats going through it. The canal must be wide enough to permit two of the(55) boats using it to(56) each other easily. It must be deep enough to leave about two feet of water(57) the keel of the largest boat using the canal. Some canals have sloping sides,(58) others have sides that are nearly vertical. Canals that are cut through rock can have nearly vertical sides. However, canals with earth banks may crumble if the angle of their sides is too steep.
Some canals are lined with brick, stone, or concrete to keep the water(59) soaking into the mud. This also permits ships to go(60) greater speeds, since they cannot make the banks fall in by stirring up the water. In small canals with mud banks, ships and barges must(61) their speed.
When the canal goes(62) different levels of water, the ships must be raised or lowered from one level to the other, This is generally done by means of locks. If a ship wants to go up to higher water, the lower end of the lock opens to let the boat in. Then this gate closes, and the water is let into the lock chamber from the upper level. This raised the level of the water in the lock(63) it is the same as the upper level of water.
Now the upper gates can be opened to(64) the ship into the higher water. Of course there must always be enough water on the upper level to allow for the flooding of the lock. Sometimes a canal contains a(65) of locks when the difference in levels is very great.
选项 A、though
B、until
C、for
D、therefore
答案B
解析 前句说，“闸中的水位一直要持续升高”，后句说，“水位平行了”，这里应该用直到(until)，强调过程。
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