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  3. Water is the oldest form of transport. The original sailing vessels were replaced by steamboats in the early 1800s and by diese

Water is the oldest form of transport. The original sailing vessels were replaced by steamboats in the early 1800s and by diese

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问题     Water is the oldest form of transport.  The original sailing vessels were replaced by steamboats in the early 1800s and by diesel power in the 1920s. A distinction is generally made between deepwater and navigable inland water transport. Domestic commerce centers on tile Great Lakes, canals, and navigable rivers.
    In 1975 water transport accounted for 22.6 percent of total intercity tonnage. Its relative share of intercity tonnage was 31.3 percent in 1947 and 31.7 percent in 1958. Tonnage declined to 27.9 percent in 1965 but increased by 1970 to 28.4 percent. This short-term increase did not stabilize. Market share dropped by 5.8 percent by 1975. Forecasted market share by 1985 is 18.4 percent of total intercity tonnage. The water transport share of revenue has been less than 2 percent of intercity freight revenue since 1955.
    The exact miles of improved waterways in operation depend in part on whether coastwise and intercostal shipping are included.  Approximately 26,000 miles of improved inland waterways were operated in 1975. Fewer miles of improved inland waterways exist than of any other transportation mode.
    The main advantage of water transport is the capacity to move extremely large shipments. Deepwater vessels are restricted in operation, but diesel-towed barges have a fair degree of flexibility. In comparison to rail and highway, water transport ranks in the middle with respect to fixed cost. The fixed cost of operation is greater than that of motor carriers but less than that of railroads. The main disadvantage of water is the limited degree of flexibility and the low speeds of transport.  Unless the source and destination of the movement are adjacent to a waterway, supplemental haul by rail or truck is required. The capability of water to transport large tonnage at low variable cost places this mode of transport in demand when low freight rates are desired and speed of transit is a secondary consideration.
    Freight transported by inland water leans heavily to mining and basic bulk commodities, such as chemicals, cement, and selected agricultural products. In addition to the restrictions of navigable waterways, terminal facilities for bulk arid dry cargo storage and load-unload devices limit the flexibility of water transport. Labor restrictions on loading and unloading at dock level create operational problems and tend to reduce the potential range of available traffic. Finally, a highly competitive situation has developed between railroads and inland water carriers in areas where parallel routings exist.
    Inland and Great Lakes water transport will continue to be a viable alternative for future logistical system design. The full potential of the St.  Lawrence Seaway has not yet been realized with respect to domestic freight. The slow passage of inland river transport can provide a form of warehousing in transit if fully integrated into overall system design. Improvements in ice-breaking equipment appear on the verge of eliminating the seasonal limitations of water transport.  
According to the passage, diesel-towed barges differ from deepwater vessels in that the former
选项 A、are less restricted in operation
B、are less flexible
C、can move larger shipments
D、are most flexible
答案A
解析 柴油机驱动的驳船与深水船只的区别在于前者的运行不怎么受限制。作者在第四段说，深水船只的运行受到限制，但是柴油驱动的驳船则有相当大的灵活性。
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