[单项选择]Although numbers of animals in a given region may fluctuate from year to year, the fluctuations are often temporary and, over long periods, trivial. Scientists have advanced three theories of population control to account for this relative constancy.
The first theory attributes a relatively constant population to periodic climatic catastrophes that decimate populations with such frequency as to prevent them from exceeding some particular limit. In the case of small organisms with short life cycles, climatic changes need not be catastrophic: normal seasonal changes in photoperiod (daily amount of sunlight), for example, can govern population growth. This theory—the density independent view—asserts that climatic factors exert the same regulatory effect on population regardless of the number of individuals in a region.
A second theory argues that population growth is primarily density-dependent—that is, the rate of growth of a population in a region decreases as the number o
A. complete indifference.
B. qualified acceptance.
C. skeptical amusement.
D. perplexed astonishment.

[单项选择]Although numbers of animals in a given region may fluctuate from year to year, the fluctuations are often temporary and, over long periods, trivial. Scientists have advanced three theories of population control to account for this relative constancy.
The first theory attributes a relatively constant population to periodic climatic catastrophes that decimate populations with such frequency as to prevent them from exceeding some particular limit. In the case of small organisms with short life cycles, climatic changes need not be catastrophic: normal seasonal changes in photoperiod (daily amount of sunlight), for example, can govern population growth. This theory—the density independent view—asserts that climatic factors exert the same regulatory effect on population regardless of the number of individuals in a region.
A second theory argues that population growth is primarily density-dependent—that is, the rate of growth of a population in a region decreases as the number of an
A. Famine.
B. Tornadoes.
C. The number of predators.
D. The variety of food supply.

[填空题]

If visitors provide food for the animals, this may upset the animals’

[填空题]If visitors provide food the animals, this may upset the animals’ ______.

[单项选择]

Although a given waveform may contain frequencies over a very broad range, as a practical matter any transmission system will be able to accommodate only a limited band of (86) . This, in turn, limits the data rate that can be carried on the transmission (87) A square wave has an infinite number of frequency components and hence an infinite (88) However, the peak amplitude of the kth frequency component, kf, is only l/k, so most of the (89) in this waveform is in the first few frequency components. In general, any digital waveform will have (90) bandwidth. If we attempt to transmit this waveform as a signal over any medium, the transmission system will limit the bandwidth that can be transmitted.

（86）处是（）
A. frequencies
B. connections
C. diagrams
D. resources

[填空题]The farmers in this region don’t use animals (do)（）farm work any more.

[单项选择]

Most animals have little connection with animals of a different kind, unless they hunt them for food. Sometimes, however, two kinds of animals come together in a partnership (伙伴关系) which is good for both of them. You may have noticed some birds sitting on the backs of sheep. This is not because they want a ride, but because they find easy food in the parasites (寄生虫)on sheep. The sheep allow the birds to do so because they remove the cause of discomfort. So although they can manage without each other, they do better together.
Sometimes an animal has a plant partner. The relationship develops until the two partners cannot manage without each other. This is so in the corals (珊瑚)of the sea. In their skins they have tiny plants which act as" dustmen", taking some of the waste products from the coral and giving in return oxygen which the animal needs to breathe. If the plants are killed, or are even prevented from receiving light so that they cannot live normally ,the
A. Some animals and plants depend on each other for existence.
B. Some animals and plants develop their relationship easily.
C. Some plants depend on each other for food.
D. Some animals live better together.