单选题 The layer of air next to the earth, which
extends upward for about 10 miles, is known
as the troposphere. On the whole, the tropo-
Line sphere makes up about 75% of all the weight
(5) of the atmosphere. It is the warmest part of
the atmosphere because most of the. solar radi-
ation is absorbed by the earth's surface, which
warms the air immediately surrounding it. A
steady decrease of temperature with increasing
(10) elevation is a most striking characteristic of
this region, whose upper layers are colder
because of their greater distance from the
earth's surface and because of the rapid radia-
tion of heat into space. (Temperatures within
(15) the troposphere decrease about 3.5° per
1,000-foot increase in altitude.) Within the
troposphere, winds and air currents distribute
heat and moisture. Strong winds, called jet
streams, are located at the upper levels of the
(20) troposphere. These jet streams are both com-
plex and widespread in occurrence. They nor-
mally show a wave-shaped pattern and move
from west to east at velocities of 150 mph, but
velocities as high as 400 mph have been
(25) noted. The influences of changing locations
and strengths of jet streams upon weather
conditions and patterns are no doubt consid-
erable. Current intensive research may even-
tually reveal their true significance.

单选题 It can be inferred from the passage that a jet plane will usually have its best average rate of speed on its run from
A. New York to San Francisco
B. Los Angeles to New York
C. Boston to Miami
D. Bermuda to New York
E. London to Washington, DC

单选题 It can be inferred from the passage that at the top of Jungfrau, which is 12,000 feet above the town of Interlaken in Switzerland, the temperature is usually
A. below freezing
B. about 42° colder than on the ground
C. warmer than in Interlaken
D. affected by the ionosphere
E. about 75° colder than in Interlaken

单选题 The passage states that the troposphere is the warmest part of the atmosphere because it
A. is closest to the sun
B. contains electrically charged particles
C. radiates heat into space
D. has winds and air currents that distribute the heat
E. is warmed by the earth's heat