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 mak

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

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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 根据句意推断，水闸里的水位应一直保持上升直到和运河上层的水位一致，故答案为[B]until。
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