阅读下面这篇短文，短文后有2项测试任务；(1)第23～26题要求从所给的6个选项中为第2～5段每段选择1个正确的小标题，(2)第27～30题要求从所给的6个选项中选择4个正确选项，分别完成每个句子。
Atmosphere
1．The atmosphere is a mixture of several gases. There are about ten chemical elements which remain permanently in gaseous form in the atmosphere under all natural conditions. Of these permanent gases, oxygen makes up about 21 percent and nitrogen about 78 percent. Several other gases, such as argon (氩), carbon dioxide, hydrogen, neon (氖), krypton (氪), and xenon (氙), comprise the remaining one percent of the volume of dry air. The amount of water vapor, and its variations in amount and distribution is of extraordinary importance in weather changes. Atmospheric gases hold in suspension (悬浮) great quantities of dust, pollen (花粉), smoke, and other impurities which are always present in considerable, hut variable amounts.
2．The atmosphere has no definite upper limits but gradually thins until it becomes imperceptible (感觉不到的). Until recently it was assumed that the air above the first few miles gradually grew thinner and colder at a constant rate. It was also assumed that upper air had little influence on weather changes. Recent studies of the upper atmosphere, currently being conducted by earth satellites and missile probings, have shown these assumptions to be incorrect. The atmosphere has three well-defined strata (layers).
3．The layer of the air next to the earth, which extends upward for about ten miles, is known as the tropophere (对流层). On the whole, it makes up about 75 per cent of all the weight of the atmosphere. It is the warmest part of the atmosphere because most of the solar radiation 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. The upper layers are colder because of their greater distance from the earth’s surface and rapid radiation of heat into space. The temperatures within the troposphere decrease about 3.5 degrees per 1,000 feet increase in altitude (海拔高度). 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 complex and widespread in occurrence. They normally show a waveshaped pattern and move from west to east at velocities (速度) 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 considerable. Current intensive research may eventually rebel their true significance.
4．Above the troposphere to a height of about 50 miles is a zone called the stratosphere (高温层). The stratosphere is separated from the troposphere by a zone of uniform temperatures called the tropopause (对流层顶). Within the lower portions of the stratosphere is a layer of ozone (臭氧) gases which filters (过滤) out most of the ultraviolet rays from the sun. The ozone layer varies with air pressure. If this zone were not there, the full blast of the sun’s ultraviolet light would burn out skins, blind our eyes, and eventually result in our destruction. Within the stratosphere, the temperature and atmospheric composition are relatively uniform.
5．The layer upward of about 50 miles is the most fascinating but the least known of the three strata. It is called the ionosphere (电离层) because it consists of electrically charged particles called ions, thrown from the sun. The northern lights (aurora borealis) originates within this highly charged portion of the atmosphere. Its effect upon weather conditions if any, is as yet, unknown. It has been proved that the upper atmosphere above certain distance continually grew thinner and colder ______．