It is sometimes said that there is no such thing as the so-called "scientific method"; the

re are only the methods used in science. Nevertheless, it seems clear that there is often a special sequence of procedures which is involved in the establishment of the working principles of science. This sequence is as follows: (1) a problem is recognized, and as much information as appears to be relevant is collected; (2) a solution(i.e. a hypothesis) is proposed and the consequences arising out of this solution are deduced: (3) these deductions are tested by experiment, and as a result the hypothesis is accepted, modified or discarded.

As an illustration of this we can consider the discovery of air pressure. Over two thousand years ago, men discovered a method of raising water from one level to another by means of the vacuum pump. When, however, this machine passed into general use in the fifteenth and sixteenth centuries, it was not possible to raise water vertically more than about 35 feet. Why？ Galileo, amongst others, recognized the problem, but failed to solve it.

The problem was then attacked by Torricelli. Analogizing(作类比推理) from the recently-discovered phenomenon of water-pressure, he postulated(假定) that a deep "sea of air" surrounded the earth; it was, he thought, the pressure of this sea of air that pushed on the surface' of the water and caused it to rise in the vacuum tube of a pump. A hypothesis, then, was formed. The next step was to deduce the consequences of the hypothesis. Torricelli reasoned that this "air pressure" would be unable to push a liquid heavier than water as high as 35 feet, and that a column of mercury(水银), for example, which weighed about 14 times more than water, would rise to only a fourteenth of the height of water, i.e. approximately 2.5 feet. He then tested this deduction by means of the experiment we all know, and found that the mercury column measured the height predicted. The experiment therefore supported the hypothesis. A further inference was drawn by Pascal, who reasoned that if this "sea of air" existed, its pressure at the bottom(i.e. sea level) would be greater than its pressure further up, and that therefore the height of the mercury column would decrease in proportion to the height above sea-level. He then carried the mercury tube to the top of a mountain and observed that the column fell steadily as the height increased, while another mercury column at the bottom of the mountain remained steady. This further proof not only established Torricelli's hypothesis more securely, but also demonstrated that, in some aspects, air behaved like water.

Why does the author mention the discovery of air pressure？

A．To show that air behaves like water in some aspects.

B．To illustrate the special sequence of procedures involved in the establishment of the working laws of science.

C．To indicate that it was difficult to use scientific methods to solve the problem of vacuum pump.

D．To show how scientists make experiments to test their deductions.