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。
Some Knowledge in Physics
1．The sun is the source of most of heat known to us. As a direct source of heat, the sun maintains life upon this planet, 150,000,000 kilometers distant. It is also an indirect source of heat. Since the earth is derived from the sun, it is to the latter that we must trace some of the earth’s internal heat that reveals itself through volcanoes, geysers (喷泉),and hot springs.
2．An insect is not afraid of gravity, but it is in deadly fear of another force of nature. This force is called surface tension (张力). A man coming out of a bath carries with him a film of water a bout one fiftieth of an inch in thickness. This weighs about a pound. A wet mouse has to carry its own weight in water. A wet fly has to lift many times its own weight, and, as everyone knows, a fly once wetted by water or any other liquid is in a very serious position indeed. An insect going for a drink is in as great danger as a man leaning over the edge of a cliff in search of food. If it once falls into the grip of the surface tension of the water--that is to say, gets wet--it is likely to remain so until it drowns. A few insects contrive to be unwettable; the majority keep away from their drink by means of long proboscis (喙).
3．When the temperature of a liquid is raised enough, the liquid boils. This means that bubbles of vapor, containing millions of molecules form below the surface. In order for such bubbles to be produced, the pressure of the vapor inside them must be equal to the pressure of the air upon the surface of the liquid. If the air pressure is greater, the bubble will collapse. The boiling point of a liquid, then, is the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure upon the liquid at the surface.
4．Solids also expand with increase in temperature and contract when cooled. But they do not behave quite so uniformly (相同地) in this respect as liquids and gases do, particularly in the case of wide variations, in temperature. Most solids expand or contract by a definite amount for every degree of the temperature that rises or falls. The amount by which no substance expands and contracts for one degree is not usually the same as that for a different substance. Thus, for a given rise in temperature, a piece of brass expands a little more than a piece of copper and much more than a piece of steel of the same size.
5．Electric trains have many advantages over those drawn by steam engines. There is no smoke to soil the passengers’ clothes and the cushions in the train. Because they carry on heavy loads of coal and water, these trains can start and stop with less waste of power. In a station they are silent: there’s no steam to produce noise to deafen the passengers. when the air pressure is greater, ______.