52. By calling it an"informal empire"(the second sentence in paragraph 2),these eant that the U. s A. had not become a real power yet B. had not occupied enough territory abroad C. was not yet economically strong enough D, carried out a policy of economic aggression 53. The Roosevelt example is used to A. support the traditional view about the U. s B. refute the view of the recent "revisionists D, support Roosevelt s foreign policy 4. The expression "risk being eclipsed on the world stage" (the last sentence in paragraph 3)mear wered by other nations B. risk being invaded by other countries tries economicall D. risk being many colonies Researchers have known for years that mammals have a"clock"somewhere in their br ontrol center that regulates circadianrhythm. Circadian rhythems are the bodys daily biological cycles, controlling such activities as sleep and wakefulness. In November 1988 neuroscientists at the University of Virginia in charlot teville reported that the suprachiasmatic nucleus(SCN), a small cluster of cells in the middle of the brain that neuroscientists have long suspected of playing a role in daily rhythms, is he long sought biological clock. The investigators performed a series of tissue transplant experiments with hamsters Some of the animals were normal hamsters, which have a circadian cycle of 24 hours; the others were mutant (genetically altered)hamsters whose cyles were only 20 or 22 hours long. the scientists first removed the sCn from the brains of the hamsters in both groups. They then implanted an scn from a mutanthamster into the brain of each normal animal, and vice versa. When the hamsters had recovered from their operations, the circadian rhythms of the two groups were reversed: The normal hamsters had a daily cycle of 20 to 22 hours, and the mutants were on The identification of the internal clock is an important seeppingstone in understanding the basic, automatic mechanisms of brain funciton. It may also help in finding ways to relive human sleep disorders 55. Before the year 1988, neuroscientists A. knew little about the location of the biological clock B. had been searching for the clock for a long time C. never thought of looking for the clock in the brain D. thought that the clock was located in body tissues52. By calling it an“informal empire”(the second sentence in paragraph 2), these revisionists meant that the U.S. _____ . A．had not become a real power yet B．had not occupied enough territory abroad C．was not yet economically strong enough D．carried out a policy of economic aggression 53. The Roosevelt example is used to _____ . A．support the traditional view about the U.S B．refute the view of the recent“revisionists” C．refute the Monroe Doctrine D．support Roosevelt's foreign policy 54. The expression“risk being eclipsed on the world stage”(the last sentence in paragraph 3)means _____ . A．risk being overpowered by other nations B．risk being invaded by other countries C．risk being left behind other countries economically D．risk being many colonies Passage 2 Researchers have known for years that mammals have a“clock” somewhere in their brainsa control center that regulates circadianrhythms. Circadian rhythems are the body's daily biological cycles, controlling such activities as sleep and wakefulness. In November 1988, neuroscientists at the University of Virginia in charlotteville reported that the suprachiasmatic nucleus(SCN), a small cluster of cells in the middle of the brain that neuroscientists have long suspencted of playing a role in daily rhythms, is he long sought biological clock. The investigators performed a series of tissue transplant experiments with hamsters. Some of the animals were normal hamsters, which have a circadian cycle of 24 hours; the others were mutant(genetically altered)hamsters whose cyles were only 20 or 22 hours long. the scientists first removed the SCN from the brains of the hamsters in both groups. They then implanted an SCN from a mutanthamster into the brain of each normal animal, and vice versa. When the hamsters had recovered from their operations, the circadian rhythms of the two groups were reversed: The normal hamsters had a daily cycle of 20 to 22 hours, and the mutants were on a 24 hour cycle. The identification of the internal clock is an important seeppingstone in understanding the basic, automatic mechanisms of brain funciton. It may also help in finding ways to relive human sleep disorders. 55. Before the year 1988, neuroscientsts _____ . A．knew little about the location of the biological clock B．had been searching for the clock for a long time C．never thought of looking for the clock in the brain D．thought that the clock was located in body tissues