56.  Gortland has long been narrowly self-sufficient in both grain and meat．However, as per capita income in Gortland has risen toward the world average，per capita consumption of meat has also risen toward the world average，and it takes several pounds of grain to produce one pound of meat. Therefore，since per capita income continues to rise，whereas domestic grain production will not increase，Gortland will soon have to import either grain or meat or both．

Which of the following is an assumption on which the argument depends?

(A) The total acreage devoted to grain production in Gortland will not decrease substantially．

(B)  The population of Gortland has remained relatively constant during the country's years of growing prosperity.

(C)  The per capita consumption of meat in Gortland is roughly the same across all income levels．

(D)  In Gortland, neither meat nor grain is subject to government price controls．

(E)  People in Gortland who increase their consumption of meat will not radically decrease their consumption of grain．

答案是E，我觉得E的假设是一个必要的前提，但是还有个前提是人口不能大幅度下降，如果下降了，那么同样可以在保持人均收入增长的情况下，消费的肉和谷物不需要进口。所以B也应该是正确的，B不够完备，如果是说成人口保持不变或者人口增长了，就是正确的了，但是人口不变已经可以提供一个充分的条件了。不知道我的理解对不对，我看了og后面的解释，我觉得说得比较莫名其妙。

同问！！！！！！！