单项选择题

Calhoun stabilized the rat population ______.

A．when it was double the number that could live in the enclosure without stress
B．by removing young rats
C．so that there was a constant number of adult rats in the enclosure
D．all of the above are correct

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单项选择题

The author implies in this passage that ______.

A．Rivera has always been objective in reporting what he finds
B．Rivera never tries to conceal his own compassion in his reporting
C．Rivera has changed people's views on the disabled
D．Rivera's work and efforts have greatly improved the conditions of the disabled in New York

单项选择题

Which of the following inferences CANNOT be made from the information in the first paragraph

A．Calhoun's experiment is still considered important today.
B．Overpopulation causes pathological behavior in rat populations.
C．Stress does not occur in rat communities unless there is overcrowding.
D．Calhoun had experimented with rats before.