[单项选择]
阅读下面的短文，文中有15处空白，每处空白给出4个选项。请根据短文的内容，从4个选项中选择1个最佳答案。

{{B}}Canal{{/B}}

Today, most countries in the world have canals. Even in the twentieth century, goods can be moved more cheaply by boat {{U}} (51) {{/U}} by any other means of trans- port. Some canals, such as the Suez or the Panama, {{U}} (52) {{/U}} 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 {{U}} (53) {{/U}} 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 {{U}} (54) {{/U}} of a canal depends on the kind of boats going through it. The canal must be wide enough to permit two of the {{U}} (55) {{/U}} boats using it to {{U}} (56) {{/U}} each other easily. It must be deep enough to leave about two feet of water {{U}} (57) {{/U}} the keel of
A. as
B. while
C. since
D. but