单项选择题
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 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 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 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 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 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.
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