1997年8月托福考试阅读理解全真试题
As a scuba diver descends， the pressure of nitrogen in the lungs increases. Nitrogen then diffuses from the lungs to the blood and from the blood to body tissues. The reverse occurs when the diver surfaces； the nitrogen pressure in the lungs falls and the nitrogen diffuses from the tissues into the blood and from the blood into the lungs. If the return to the surface is too rapid， nitrogen in the tissues and blood cannot diffuse out rapidly enough and nitrogen bubbles are formed . They can cause severe pains， particularly around the joints.
Another complication may result if the breath is held dur- ing ascent. During ascent from a depth of 10 meters， the volume of air in the lungs will double because the air pressure at the surface is only half of what it was at 10 meters. This change in volume may cause the lungs to distend and even rup- ture. This condition is called air embolism. To avoid this event， a diver must ascent slowly， never at a rate exceeding the rise of the exhaled air bubbles， and must exhale during ascent.
21.What does the passage mainly discuss？
（A） The equipment divers use
（B） The effects of pressure on gases in the human body
（C） How to prepare for a deep dive
（D） The symptoms of nitrogen bubbles in the bloodstream.
22.The word "exposed to" in line 8 are closest in meaning to
（A） leaving behind
（B） prepared for
（C） propelled by
（D） subjected to
23.The word "exert" in line 21 is closest in meaning to
（A） cause
（B） permit
（C） need
（D） change
24.The word "diffuses" in line 23 is closest in meaning to
（A） yields
（B） starts
（C） surfaces
（D） travels
25.What happens to nitrogen in body tissues if a diver ascends too quickly.
（A） It forms bubbles
（B） It goes directly to the brain
（C） It is reabsorbed by the lungs.
（D） It has a narcotic effect.
26.The word "They" in line 29 refers to
（A） joints
（B） pains
（C） bubbles
（D） tissues.
27.The word "rupture" in line 36 is closest in meaning to
（A） hurt
（B） shrink
（C） burst
（D） stop
28.It can be inferred from the passage that which of the following presents the greatest danger to a diver？
（A） Pressurized helium
（B） Nitrogen diffusion
（C） Nitrogen bubbles
（D） An air embolism
29.What should a diver do when ascending？
（A） Rise slowly
（B） Breathe faster
（C） Relax completely
（D） Breathe helium.
Question 29-38
Each advance in microscopic technique has provided scientists with new perspectives on the function of living organisms and the nature of matter itself. The invention of the visible-light microscope late in the sixteenth century introduced a previously unknown realm of single-celled plants and animals. In the twentieth century， electron microscopes have provided direct views of viruses and minuscule surface structures. Now another type of microscope， one that utilize x-rays rather than light or electrons， offers a different way of examining tiny details， it should extend human perception still farther into the natural world.
The dream of building an x-ray microscope dates to 1895， its development， however， was virtually halted in the 1940 s because the development of the electron microscope was progressing rapidly. During the 1940 s electron micro- scopes routinely achieved resolution better than that possible with a visible-light microscope， while the performance of x-ray microscopes resisted improvement. In recent years， however，interest in x-ray microscopes has revived， largely because of advances such as the development of new sources of x-ray illumination. As a result， the brightness available today is millions of times that of x-ray tubes， which， for most of the century， were the only available sources of soft x-rays.
The new x-ray microscopes considerably improve on the resolution provided by optical microscopes. They can also be used to map the distribution of certain chemical elements. Some can form pictures in extremely short times， others hold the promise of special capabilities such as three dimensional imaging. Unlike conventional electron microscopy， x-ray microscopy enables specimens to be kept in air and in water，which means that biological samples can be studied under conditions similar to their natural state. The illumination used，so-called soft x-rays in the wavelength range of twenty to forty angstroms （an angstrom is one ten-billionth of a meter）， is also sufficiently penetrating to image intact biological cells in many cases. Because of the wavelength of the x-rays used，soft x-ray microscopes will never match the highest resolution possible with electron microscopes. Rather， their special pro- perties will make possible investigations that will complement those performed with light- and electron-based instruments.