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.
36. The word they in the passage
refers to
a.deserts
b.strong winds
c.sand grains
d.structures
答案是c.
按照文法來說，我也選C
更進一步我想請教的是they是當upon landing的受詞還是當dislodge的主詞？
不知到這樣問會不會太淺，顯示自己文法爛
