[单选题]Not many years ago, some farmers were worried because hawks were taking many of their chicken. The farmers didn't know what to do. Finally they went to the country officials and asked for help. “Kill the hawks,” the officials said, “We will even pay for them,” so the farmers began to think of ways to kill the hawks.The farmers killed many hawks. They no longer had to worry about their chickens. But they now had a new worry. Field mice were eating up a lot of the farmers’grain. How did this happen? Hawks eat not only chickens but also field mice. They eat more field mice than chickens. But the farmers didn't know this. When they killed a lot of hawks, they changed the balance. When people move into a new place, they often destroy many wild plants. Often these plants are food for the animals. If the animals can‘t find enough plants to eat, they will starve or have to leave the place.In one part of the USA, for example the deer there like to eat a certain kind of wild roses. The mountain lions there eat the deer. The number of deer, mountain lions and wild roses doesn't change much if people leave things as they are. But people killed many mountain lions in order to protect the deer. Soon there were so many deer that they ate up all the wild roses. Then the deer began to eat the green leaves of young trees. These trees were important to the farmers. So the farmers thought of ways to protect their trees. Now the deer had nothing to eat, and many of them died. This was another lesson from nature.To keep the balance of nature is important for us to remember.The farmer thought of many ways to prevent deer from eating up wild roses. A True B False C Not given D E F