阅读 ppreview test1 42of 34-42

ball2003

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.  arabolic 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.

42. Which of the following is probably

   most influential in determining the

   shape of parabolic dunes?

   Moisture  

   Vegetation  

   Wind speed  

   Soil composition  

请大家给我指点，我这题作不出答案啊？！谢谢。

[此贴子已经被作者于2004-11-15 3:34:05编辑过]

漂亮宝贝

B:Vegetation

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.

icepear

你真牛，打这么多字。

我也认为是B，原文是这样，P沙丘在这样一个地方形成，那有稀少的植被固定了周围，而中间的土壤却外露了，导致中间的沙前进。

ball2003

谢谢大家，我现在明白了。

icepear,我不是自己输入的啊，有PP REVIEW的word 文件可以粘贴的。