Today, most countries in the world have cs. Many countries have built cs 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 waterways make it (69) for boats to travel (70) ports along the coast without being (71) to the ers of the open. Some cs, such as the Suez and the Panama, save ships weeks of time by their (72) a thousand miles shorter. Other cs permit boats to reach cities that are not (73) on the coast, still other cs (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 c (77) on the kind of boats going through it. The c 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 (船的龙骨) of the largest boat using the c. When the planet Mars was first (80) through a telescope, people saw that the round disk of the planet was crisscrossed by a (81) of strange blue-green lines. These were called "cs" (82) they looked the same as cs on earth (83) are viewed from an airplane. However, scientists are now (84) that the Martian phenomena are really not cs. The photographs (85) from space-ships have helped us to (86) the truth about the Martian "cs".