专四专八考试

【单选题】About ten percent of the world’s deserts are composed of sand dunes, which are driven across the desert by the winD、Sand grains march across the desert floor under the influence of strong winds by a process known as saltation. The grains of sand become airborne for a moment, and upon landing they dislodge additional sand grains, which repeat the process. In this manner, sand dunes engulf everything in their path, including structures made by people, and pose a major problem in the construction and maintenance of highways and railroads that cross sandy areas of desert. Sand dune migration near desert oases poses another serious problem, especially when encroaching on villages. Methods to mitigate damage to structures from sand dunes include building windbreaks and funneling sand out of the way. Without such measures, disruption of roads, airports, agricultural settlements, and towns could become a major problem in desert regions.
The direction, strength, and variability of the wind, the moisture content of the soil, the vegetation cover, the underlying topography, and the amount of movable soil exposed to the wind determine the size and form of sand dunes. Sand dunes generally have four basic shapes, determined by the topography of the land and patterns of wind flow. Linear dunes align in roughly the direction of strong prevailing winds. Their length is substantially greater than their width, and they lie parallel to each other, sometimes with a wavy pattern.Crescent dunes, also called barchans, are symmetrically shaped, with horns pointing downwind, They travel across the desert at speeds of up to 50 feet a year. Parabolic dunes form in areas where sparse vegetation anchors the side arms while the center is blown outward, causing sand in the middle to move forwarD、Star dunes form by shifting winds that pile up sand into central points that can rise 1,500 feet and more, with several arms radiating outward, looking much like giant pinwheels.
According to the passage, in what way do sand dunes interfere with transportation
A、They cause travelers to get lost.
B．They make it difficult to build and repair roadways.
C．They increase the likelihood of accidents.
D．They block the most direct routes.

网考网参考答案：B
网考网解析：
事实细节题。由第一段可知，沙丘会吞没道路上的任何东西，包括各种建筑物，从而对通过沙漠地区的道路的建筑和维护带来不小的问题。 document.getElementById("warp").style.display="none"; document.getElementById("content").style.display="block"; 查看试题解析出处>>