Scientists have modified feed corn genetically, increasing its resistance to insect pests. Farmers who tried out the genetically modified corn last season applied less insecticide to their corn fields and still got yields comparable to those they would have gotten with ordinary corn. Ordinary corn seed, however, costs less, and what these farmers saved on insecticide rarely exceeded their extra costs for seed. Therefore, for most feed-corn farmers, switching to genetically modified seed would be unlikely to increase profits.
Which of the following would it be most useful to know in order to evaluate the argument?
A. Whether there are insect pests that sometimes reduce feed-corn yields, but against which commonly used insecticides and the genetic modification are equally ineffective
B. Whether the price that farmers receive for feed corn has remained steady over the past few years
C. Whether the insecticides typically used on feed corn tend to be more expensive than insecticides typically used on other crops
D. Whether most of the farmers who tried the genetically modified corn last season applied more insecticide than was actually necessary
E. Whether, for most farmers who plant feed corn, it is their most profitable crop
关于d选项：不是和文中已知信息重复了吗，文中已经说杀虫剂的使用会变少。
b选项：考察价格，如果今年价格变高那么在成本增加的情况下利润是有可能比去年多的不是吗？那就可以削弱结论。反之是增强，b选项为什么不可以。