单选题 Today, most countries in the world have canals. Many countries have built canals near the coast, and parallel 1 the coast. Even in the twentieth century, goods can be moved more cheaply by boat than by any other 2 of transport. These 3 make it possible for boats to travel 4 ports along the coast without being 5 to the dangers of the open. Some canals, such as the Suez and the Panama, save ships weeks of time by making their 6 a thousand miles shorter. Other canals permit boats to reach cities that are not 7 on the coast, still other canals 8 lands where there is too much water, help to 9 fields where there is not enough water, and 10 water power for factories and mills. The size of a canal 11 on the kind of boats going through it. The canal must be wide enough to permit two of the largest boats using it to 12 each other easily. It must be deep enough to leave about two feet of water 13 the keel of the largest boat using the canal. When the planet Mars was first 14 through a telescope, people saw that the round disk of the planet was criss-crossed by a 15 of strange blue-green lines. These were called "canals" 16 they looked the same as canals on earth 17 are viewed from an airplane. However, scientists are now 18 that the Martian phenomena are really not canals. The photographs 19 from spaceships have helped us to 20 the truth about the Martia "canals".