【单选题】

Of the 197 million square miles making up the surface of the globe, 71 percent is covered by the interconnecting bodies of marine water; the Line Pacific Ocean alone covers half theEarth and

(5) averages nearly 14,000 feet in depth. The continents-Eurasia,Africa, NorthAmerica, SouthAmerica,Australia, andAntarctica—are the portions of the continental masses rising above sea level. The submerged borders of the continental
(10) masses are the continental shelves, beyond which lie the deepsea basins.
The oceans attain their greatest depths not in
their central parts, but in certain elongated fur-
rows, or long narrow troughs, called deeps. These
(15) profound troughs have a peripheral arrangement,
notably around the borders of the Pacific and
Indian oceans. The position of the deeps near the
continental masses suggests that the deeps, like
the highest mountains, are of recent origin, since
(20) otherwise they would have been filled with waste
from the lands. This suggestion is strengthened by
the fact that the deeps are frequently the sites of
world-shaking earthquakes. For example, the
"tidal wave" that inApril, 1946, caused wide-
(25) spread destruction along Pacific coasts resulted
from a strong earthquake on the floor of the
AleutianDeep.
The topography of the ocean floors is none too
well known, since in great areas the available
(30) soundings are hundreds or even thousands of
miles apart. However, the floor of theAtlantic is
becoming fairly well known as a result of special
surveys since 1920.A、broad, well-defined
ridge—the Mid-Atlantic ridge—runs north and
(35) south betweenAfrica and the twoAmericas, and
numerous other major irregularities diversify the
Atlantic floor.Closely spaced soundings show
that many parts of the oceanic floors are as
rugged as mountainous regions of the continents.
(40) Use of the recently perfected method of echo
sounding is rapidly enlarging our knowledge of
submarine topography.During World War Ⅱ
great strides were made in mapping submarine
surfaces, particularly in many parts of the vast
(45) Pacific basin.
The continents stand on the average 2870
feet—slightly more than half a mile—above sea
level. NorthAmerica averages 2300 feet;Europe
averages only 1150 feet; andAsia, the highest
(50) of the larger continental subdivisions, averages
3200 feet. The highest point on the globe, Mount
Everest in the Himalayas, is 29,000 feet above the
sea; and as the greatest known depth in the sea
is over 35,000 feet, the maximum relief (that is,
(55) the difference in altitude between the lowest and
highest points) exceeds 64,000 feet, or exceeds 12
miles. The continental masses and the deep-sea
basins are relief features of the first order; the
deeps, ridges, and volcanic cones that diversify
(60) the sea floor, as well as the plains, plateaus, and
mountains of the continents, are relief features of
the second order. The lands are unendingly sub-
ject to a complex of activities summarized in the
term erosion, which first sculptures them in great
(65) detail and then tends to reduce them ultimately to
sea level. The modeling of the landscape by
weather, running water, and other agents is appar-
ent to the keenly observant eye and causes think-
ing people to speculate on what must be the final
(70) result of the ceaseless wearing down of the land