The "balance of nature" is not an empty phrase. Nature provides a population to occupy a s

uitable environment and cuts down surplus population to fit the available food supply. One means of reducing surplus population is predators(食肉动物); others are parasites(寄生虫)and diseases. Also, population density produces nervous disorders and even drives animals to mass migrations, like the lemmings(旅鼠)of norway who plunge(跳入)into the sea. That predators populations increase to control other animals has long been known. Many years ago, the Hudsons Bay Company records revealed that the fox population went up and down about a year after the rabbit population had gone up and down. Sometimes a situation occurs in which the predators population is reduced to a level below that which nature can readily replace. On Valcour Island in Lake Champlain(New York), a costly campaign resulted in the elimination of predatory animals only to have birds and small animals(including grouse(松鸡)and hares, popular game)increase for four years afterward. Then, lacking predator control, nature resorted to disease to cut down these populations. Jamaica had an example of natures persistence in providing animals for existing habits. Sugar planters, about 75 years ago, imported mongooses(獴)to control rats. The mongooses killed off the rats and, with plentiful food, multiplied. Rats became scarce and the mongooses ate lambs, kids, puppies(小狗), and wildlife. Eventually, food became scarce and the mongooses population declined.

The phrase "balance of nature"(Line 1, Para. 1)means______.

A．reducing the population of predators

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