资料题：It is no easy job to carry out a general plan for birth control throughout the world because（  ）。

The advantages and disadvantages of a large population have long been a subject of discussion among economists.It has been argued that the supply of good land is limited.To feed a large population, inferior land must be cultivated and the good land worked intensively.Thus,each person produces less, which means a lower average income than could be obtained with a smaller population.Other economists have argued that a large population gives more scope for specialization and the development of facilities such as ports, roads and railways, which are not likely to be built unless there is a big demand to justify them.One of the difficulties in carrying out a world-wide birth control program lies in the fact thatofficial attitudes to population growth vary from country to county, depending on the level of industrial development and the availability of raw materials.In a developing county where a vastly expanded population is pressing hard upon the limits of food, space and natural resources, it will be the first concern of government to place a limit on the birthrate, whatever the consequences my be.In a highly industrialized society, the problem may be more complex.A decreasing birthrate may lead to unemployment because it results in a declining market for manufactured goods.When the pressure of population on housing declines, prices also decline and the building industry is weakened.Faced with considerations such as these, the government of a developed county may well prefer to see a slowly increasing population, rather than one which is stable or in decline.