单选题 Researchers have known for years that animals have a "clock" somewhere in their brains a control-centre that regulates circadian(生物节奏的) rhythms. Circadian rhythms 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 supra chasmatic nucleus(SCN), a small cluster(束)of cells in the middle of the brain that neuroscientists have long suspected of playing a role in daily rhythms, is the long-sought biological clock.
The investigators performed a series of tissue-transplant experiments with hamsters (仓鼠). Some of the animals were normal hamsters, which have a circadian cycle of 24 hours; the others were mutant(变异型)hamsters whose cycles were only 20 or 22 hours long.
The scientists first removed the SCN from the brans of the hamsters in both groups. They then implanted an SCN from a mutant hamster 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 stepping-stone in understanding the basic, automatic mechanisms of brain function. It may also help in finding ways to relieve human sleep disorders.

单选题 The purpose of the experiments was to fund out how the clock works.

单选题 the identification of the internal clock may help to relieve human sleeping problems.

单选题 The normal hamsters had a daily cycle of 24 hour, and the mutants were on a 20- hour cycle.

单选题 SCN plays a role in daily rhythms.

单选题 Scientists are cruel to do experiments with hamsters.

单选题 Biological clock tells time.

单选题 Hamsters have sleeping problems.