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。 Canal Today, most countries in
the world have canals. Even in the twentieth century, goods can be moved more
cheaply by boat (51) by any other means of trans- port. Some
canals, such as the Suez or the Panama, (52) 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
(53) 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 (54) of a canal depends on
the kind of boats going through it. The canal must be wide enough to permit two
of the (55) boats using it to (56) each
other easily. It must be deep enough to leave about two feet of water
(57) the keel of the largest boat using the canal. Some
canals have sloping sides, (58) others have sides that are
nearly vertical. Canals that are cut through rock can have nearly vertical
sides. However, canals with earth banks may crumble if the angle of their sides
is too steep. Some canals are lined with brick, stone, or
concrete to keep the water (59) soaking into the mud. This
also permits ships to go (60) greater speeds, since they
cannot make the banks fall in by stirring up the water. In small canals with mud
banks, ships and barges must (61) their speed.
When the canal goes (62) different levels of water,
the ships must be raised or lowered from one level to the other, This is
generally done by means of locks. If a ship wants to go up to higher water, the
lower end of the lock opens to let the boat in. Then this gate closes, and the
water is let into the lock chamber from the upper level. This raised the level
of the water in the lock (63) it is the same as the upper
level of water. Now the upper gates can be opened to
(64) the ship into the higher water. Of course there must
always be enough water on the upper level to allow for the flooding of the lock.
Sometimes a canal contains a (65) of locks when the
difference in levels is very great.