Three years ago, researchers announced the discovery
of human genes that were capable of turning ordinary cells into malignant ones.
The news met with some skepticism. Experts asked how a single gene could cause
such a dramatic change. Why does cancer take years or even decades to develop if
it is caused by such a simple and direct process In last week’s issue of the,
three research teams answered those questions by setting forth a new model for
understanding the role of oncogenes in cancer.
Each group found
that it does in fact take more than a single gene to produce cancer in normal
cells. Teams at M. I. T and Cold Spring Harbor Laboratory on Long Island, N. Y.
, reported that they could induce cancer in normal rat cells only by inserting
at least two types of oncogene into the cells. "A single oncogene produced some
changes,
A. was caused by normal cells
B. sometimes resulted from internal actions of the victim’s own body
C. was a hereditary disease
D. was a form of malignancy
[单项选择]What happened 25 years ago（）
A. Michael’s parents got divorced.
B. Karen’s mother died in a car accident.
C. A truck driver lost his wife in a collision.
D. Karen was adopted by Ray Anderson.
[单项选择] Passage Four
About one million years ago, the Ice
Age began. The Ice Age was a long period of time in which four great glaciers
pushed southward to cover almost all the upper half of North America, and then
melted away. Each glacier was a thick sheet of ice and snow that spread out from
a centre near what is now Hudson Bay in Canada. The winters were long, and the
cool summers were too short to melt much of the ice and snow. The ever-growing
sheet built up to a thickness of two miles at its centre.
As all
glaciers do, these great glaciers slid. They pushed down giant trees in their
paths and scraped the earth bare of soil. Many animals moved farther south to
escape. Others stayed and were destroyed.
When winters of little
snow came, the summer sun cut into the edges of the ice sheets. As the glaciers
melted, rocks, soil and other things
A. the Ice Age was a long period of time
B. great glaciers covered North America many years ago
C. changes in climate helped to melt the glaciers
D. how glaciers changed North America
[单项选择] Passage Three
Thousands of years ago man used handy
rocks for his surgical operations. Later he used sharp bone or horn, metal
knives and, more recently, rubber and plastic. And that was where we stuck, in
surgical instrument terms, for many years. In the 1960s a new tool was
developed, one which was, first of all, to be of great practical use to the
armed forces and industry, but which was also, in time, to revolutionize the art
and science of surgery.
The tool is the laser and it is, being
used by more and more surgeons all over the world, for a very large number of
different complaints. The word laser means: Light Amplification by Stimulated
Emission of Radiation. As we all know, light is hot; any source of light--from
the sun itself down to a humble match burning--will give warmth. But light is
usually spread out over a wide area. The l
A. it is a highly intensified beam of light
B. its strength is increased by the heat of the sun
C. it can be connected to strong light sources
D. it is a concentration of light from different sources
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