Cheap protein substitutes from milk, eggs and meat ______.

C
事实细节的找寻和判断。题干中的关键词为cheap protein，从而定位到录音后半部分提到的“Mixtures of vegetable proteins, like soybeans and peanuts, could provide an abundance of cheap, useful protein where meats, eggs, milk are not within economic reach…”，可知如果因为经济原因而买不起肉、蛋、奶时，蔬菜类蛋白混合物同样可以提供丰富、便宜并且有益的蛋白质，例如大豆和花生。由此可知，来自奶、蛋和肉的蛋白质可以从大豆中获得。故选项C为正确答案。