A、of little nutritional value B、difficult to grow C、a plant that exhausts the soil D、a source of protein D根据关键词正确找寻到原文相关处是解题的关键。

In most corners of the world malnutrition is plainly a matter of outright insufficiency of food for population -- where the majority of the people do not obtain enough food calories to meet minimal needs for support of physical work and for maintenance of health. Elsewhere the problem may be not one of insufficient calories but of lack of specific nutrients essential for health.
In Latin America, as in other places, the dread protein deficiency disease kwashiorkor is taking its heavy toll on children’s lives. Strategic vitamins and minerals may be lacking due to traditional diets which are nutritionally imbalanced. Here people continue their eating pattern year after year without knowledge of what their dietary habits are doing to themselves and to future generation. With a basic knowledge of nutritional needs and deficiencies, efforts would be directed to finding food substitutes which could meet these needs. Mixtures of vegetable proteins, like soybeans and peanuts, could provide an abundance of cheap, useful protein where meats, eggs, and milk are not within economic reach of large groups in the population. Efforts could also be expanded on increasing the agricultural productivity in specific regions; where large areas are given over to relatively inefficient use as grazing land, the intensive production of vegetable protein crops could bring remedial nutrition to an undernourished population.