单项选择题

Today, most countries in the world have canals. Many countries have built canals near the coast, and parallel (67) the coast. Even in the twentieth century, goods can be moved more cheaply by boat than by any other (68) of transport. These (69) make it possible for boats to travel (70) ports along the coast without being (71) to the dangers of the open. Some canals, such as the Suez and the Panama, save ships weeks of time by making their (72) a thousand miles shorter. Other canals permit boats to reach cities that are not (73) on the coast; still other canals (74) lands where there is too much water, help to (75) fields where there is not enough water, and (76) water power for factories and mills. The size of a canal (77) on the kind of boats going through it. The canal must be wide enough to permit two of the largest boats using it to (78) each other easily. It must be deep enough to leave about two feet of water (79) the keel of the largest boat using the canal. When the planet Mars was first (80) through a telescope, people saw that the round disk of the planet was crises-crossed by a (81) of strange blue-green lines. These were called "canals" (82) they looked the same as canals on earth (83) are viewed from an airplane. However, scientists are now (84) that the Martian phenomena are really not canals. The photographs (85) from space-ships have helped us to (86) the truth about the Martian "canals".