2. 外语
  3. Today, most countries in the world have canals. Some canals, such as the Suez or the Panama,【C1】______ ships weeks of time by ma

Today, most countries in the world have canals. Some canals, such as the Suez or the Panama,【C1】______ ships weeks of time by ma

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问题     Today, most countries in the world have canals. Some canals, such as the Suez or the Panama,【C1】______ 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.【C2】______ other canals drain lands where there is too much water, help or【C3】______ fields where there is not enough water.
    The【C4】______ of a canal depends on the kind of boats going through it. The canal must be wide enough to permit two of the【C5】______ boats using it to【C6】______ each other easily. It must be deep enough to leave about two feet of water【C7】______ the keel of the largest boat using the canal.
    Some canals have sloping sides, while others have sides that are nearly【C8】______ Canals that are cut through rock can have nearly Vertical sides.【C9】______ , canals with earth banks may【C10】______ if the angle of their sides is too steep. Some canals are lined with brick, stone, or concrete to keep the water【C11】______ soaking into the mud. This also permits ships to go at【C12】______ speeds, since they cannot make the banks fall in by【C13】______ up the water. In small canals with mud banks, ships and barges (大平底船) must【C14】______ their speed.
    When the canal goes【C15】______ different levels of water, the ships must be【C16】______ or lowered from one level to the other. This is generally done by【C17】______ 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 【C18】______ it is the same as the upper level of water. Now the upper gates can be opened to【C19】______ the ship into the higher water. Sometimes a canal contains a series of locks when the【C20】______ in levels is very great.  
【C18】
选项 A、after
B、until
C、by
D、during
答案B
解析 根据句意：水闸里的水位应一直保持上升直到和运河里的水位一致，这样船只才能通过，故选 B。
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