chenxiangdong逻辑推理

An acre of average farmland produces about 400 pounds of grain amaranth ,as against 2,400 pounds per acre,or six times as much,for wheat.It follows that whenever the grain-amaranth price is projected to be more than six times the projected price of wheat,farmers wishing to maximize profits will grow grain amaranth rather than wheat.

The argument above is based on which of the following assumptions?

a An acre's worth of grain amaranth is no more expensive to grow and bring to market than acre's worth of wheat.

bBy choosing which crops to grow,farmers can exert a significant influence on the prices of those crops.

答案是a。不明白为什么不选b

1.凡是问

The argument above is based on 或者depends on  which of the following assumptions?

这样的题目都是必要型的吧，如果不是，怎么才能看出一个题目是充分型还是必要型的？

an assumption of the passage is that?  这种问法是充分型的么？

2.怎么看出题目考察的是“除了隐含的假设外没有别的因素影响结论”，是结论对吧

是不得看文中有没有几个事物之间的比较？？