单项选择题

When the mongooses in Jamaica killed off the rats they ______.

A．attacked the humans
B．ate the sugar crop
C．became problems themselves
D．had nothing to eat
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单项选择题
The phrase balance of nature in the first line means ______.
A．reducing the population of.predators and parasites
B．keeping the right number of animals for the right amount of food
C．keeping the ratio of small game to predators
D．driving animals to mass migrations
单项选择题
The fact that the number of one kind of animal may have much to do with that of other animals
A．is not recent knowledge
B．has just been learned
C．has not proved yet
D．is not important now
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