Today, most countries in the world have canals. Many countries have built canals near the coast, and parallel【C1】______the coast. Even in the twentieth century, goods can be moved more cheaply by boat than by any other【C2】______of transport. These waterways make【C3】______possible for boats to travel【C4】______ports along the coast without being【C5】______to the dangers of the open. Some canals, such as the Suez and the Panama, save ships weeks of time by making their【C6】______a thousand miles shorter. Other canals permit boats to reach cities that are not【C7】______on the coast, still other canals【C8】______water from rivers or lakes to help【C9】______fields where there is not enough water, and【C10】______water power for factories and mills. The size of a canal【C11】______on the kind of boats going through it. The canal must be wide enough to .permit two of the largest boats using it to【C12】______each other easily. It must be deep enough to leave about two feet of water【C13】______the keel (龙骨) of the largest boat using the canal. When the planet Mars was first【C14】______through a telescope, people saw that the round disk of the planet was criss-crossed by a【C15】______of strange blue green lines. These were called 'canals'【C16】______they looked the same as canals on earth【C17】______are viewed from an airplane. However, scientists are now【C18】______that the Martian phenomena are really not canals. The photographs【C19】______from spaceships have helped us to【C20】______the troth about the Martian 'canals'. 【C1】