Let us consider what science and technology have to contribute to the food problem. The st way to increase food production, one might suppose, is to bring more land 【B1】 cultivation and put more people to work on it. Some of the underdeveloped countries have resorted to this 【B2】 approach, without notable success. It contains several fallacies. For one thing, it usually means 【B3】 into marginal lands where the soil and climatic conditions give a poor 【B4】. Cultivation may quickly deplete this soil, 【B5】 it for pasture or forest growth. It is often possible, of course, to turn such lands into useful farms by agricultural 【B6】; for instance, a sophisticated knowledge of how to use the available water 【B7】 an irrigation system may reclaim semi-arid grasslands for crop-growing. But the cultivation of marginal lands is in any case unsuccessful 【B8】 it is carried out by farmers with a centuries-old tradition of experience or by modern experts with a detailed knowledge of the 【B9】 conditions and the varieties of crops that are suitable for those conditions. Such knowledge is 【B10】 absent in the underdeveloped countries. 【B11】, we know that highly developed countries have not increased the 【B12】 of acres under cultivation , 【B13】 on the contrary have 【B14】 their marginal lands and steadily reduced the proportion of the population engaged in farming. Efficient farming 【B15】 concentration on the most efficient lands, and it results in greater production with 【B16】 people. The problem of the underdeveloped countries, then is to increase the 【B17】 of their farms and farmers. This would allow them to industrialize and to feed their people more adequately. It is not easy to 【B18】, however. The farmers are conservative and resistant to change 【B19】 their methods of cultivation. The underdeveloped countries are greatly in need of studies and experiments to help them to 【B20】 modem agricultural methods to their own conditions.

A．reflection
B．amount
C．return
D．output