[单项选择]Today, most countries in the world have canals. Some canals, such as the Suez or the Panama, (62) 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. (63) other canals drain lands where there is too much water, help or (64) fields where there is not enough water.
The (65) of a canal depends on the kind of boats going through it. The canal must be wide enough to permit two of the (66) boats using it to (67) each other easily. It must be deep enough to leave about two feet of water (68) the keel of the largest boat using the canal.
Some canals have sloping sides, while others have sides that are nearly (69) Canals that are cut through rock can have nearly Vertical sides. (70) , canals with earth banks may (71) if the angle of their sides is too steep. Some canals are lined with
A. after
B. until
C. by
D. during

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