| 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]A. Three years ago. B. For three years.
C. Maybe in three years. D. No, only three years. [单项选择]Not until three years ago ______ to work outside.
A. he began B. he begins C. began he D. did he begin [填空题]I first met Lisa three years ago. She (work) ______ at a radio shop at the time.
[单项选择]
Passage Three Until two years ago, Clearing, Illinois was a tranquil suburb of Chicago. But residents grew alarmed when they noticed armed teenagers on the streets, giving gang signals and shouting at passing cars. Then came a series of burglaries and graffiti messages on store- fronts. By the time local authorities realized they had a gang problem, it was too late. Last December, two 13-year-old girls were shot outside their school as they sat in a car with two members of a local gang, the Ridgeway Lords. Nearly all 50 states have recently passed laws that allow youths aged 14-17 to be tried in court as adults. In about 25 states they have passed laws to punish parents for their children’s behavior. And in 146 of the nation’s largest cities, they have imposed curfews to reduce juvenile violence. When you look at the spectacular rise of violent crime among young people recently, it’s easy to understand the concern. Over the past decade, there ha [填空题]His brother joined the army three years ago.
His brother______ ______ ______the army for three years.
[单项选择]If you have three years’ work experience, you will be the right ______ for this job.
A. person B. passenger C. tourist D. customer [单项选择]I would rather I （ ） her three years ago.
A. will not marry B. had not married C. have not married D. will not have married [单项选择]
Why did Ms. Mann go to Argentina three years ago A. She went to see her grandparents. B. She went for pleasure. C. She wrote political reports for the BBC. D. She wrote economic stories for the BBC. [单项选择]Carl studied in our high school three years ago. Last August his father found a job in another city and his family moved there. He began to study in a new school and we often write to each other. He often tells me something about the city where he lives now and his studies in the school. So I can know what happens to him.
Last Friday Carl came to our city again. He hoped to stay with his old grandpa during the summer holiday. He’s taller and stronger than before. We swam in the river outside the city together or had a picnic on the island. It surprised me that he has learned to cook when we were travelling in the wild forest. He told me his parents were both busy and sometimes he stayed at home alone and he had to look after himself. "How do you like your school" I asked. "Wonderful," said the young man. "It has a tall building and there’re two chemistry labs, two biology labs and three physics ones. And our library is big and there’re plenty of books in it." "I can A. he often visits me B. I often telephone him C. he writes to me now and then D. I live not far from him [填空题]Ms Clark came to Beijing three years ago．
→It has been______． [填空题]Ms Clark came to Beijing three years ago.
→ It has been ______.
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