求助！！！一道 很纠结的题目！！！！！

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 continue 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 year of growing properity.
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.
有哪位G友能解释下这道题的？答案选E，主要是不明白Per capita 是什么意思？ 而且为什么选E？