| 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]Passage Three
The men who planned and constructed the first American railroads were men of great vision. They predicted that their “iron horses” would open vast expanses of land to farming and industry. They knew that towns would spring up along the new railroads. Yet few of these pioneers could have guessed that American railroads would establish the system of time used throughout most of the world today.
Until the 1880s, all time in the United States was sun time. Farmers set their clocks to noon when the sun appeared to be overhead. In towns, people set their clocks by a courthouse clock or factory whistles. One town’s time was often different from the time in a neighboring town.
As the new railroads expanded, large numbers of people began to travel. But the railroads were unable to print accurate timetables when most towns along their tracks have their clocks set differently. There was need for a change, and the railroads led in planning it.
In 1883, rai
A. Farmers set their clocks according to the factory whistles B. The correct time was not too important before the 1800s C. Only people who traveled on trains ever knew time D. A standard system was set up by the pioneers [单项选择]
A. It is lessening. B. It is growing. C. It hasn’t changed. D. It is slowly changing. 我来回答: 提交
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