单项选择题

Today the game reserves(猎物保护区)of East Africa are facing a number of threats. (76)Although they earn considerable revenue(收入) by attracting tourists, they take up land which is increasingly sought by the local people. While these reserves feed and protect animals, they are in danger of turning into barren areas or deserts. Trees, shrubs and grass are gradually being eaten by grazing herds.
Another problem is to be found in the changing attitudes of the animals themselves. Many of them are losing their hereditary(天生)fear of man. In this way they may become a danger to visitors and thus to themselves. Attacks on vehicles are beginning to increase, and it is possible that the problems will become serious in a few years’ time.
The problem of shortage of land is not a simple one. (77) As the population increases, more and more people look hungrily at the land set aside for game reserves. They claim that a Government’s first duty is to its inhabitants and not to tourists or to wild animals. Despite the income obtained from tourism, this is an argument which it is difficult to answer satisfactorily.
______ is causing soil erosion(侵蚀)in the game reserves.