[求助]请教一道GWD争议较大的题6-21

以下是引用Drakedragon在2008-3-6 16:45:00的发言：
Q21:
Scientists have made genetic modifications to cotton to increase its resistance to insect pests.  According to farmers’ report, the amount of insecticide needed per acre to control insect pests was only slightly lower for those who tried the modified seed than for those who did not.  Therefore, since the modified seed costs more than ordinary seed without producing yields of higher market value, switching to the modified seed would be unlikely to benefit most cotton farmers economically.

Which of the following would it be most useful to know in order to evaluate the argument?

A Whether farmers who tried the modified cotton seed had ever tried growing other crops from genetically modified seed
B Whether the insecticides typically used on ordinary cotton tend to be more expensive than insecticides typically used on other crops
C Whether for most farmers who grow cotton it is their primary crop
D Whether the farmers who have tried the modified seed planted as many acres of cotton, on average, as farmers using the ordinary seed did
E Whether most of the farmers who tried the modified seed did so because they had previously had to use exceptionally large quantities of insecticide
请教大家已下，正解是B，我选的D，我的想法是如果原来杀虫剂用的多，则削弱结论，用的少则加强结论，请问我的想法错在哪里？其次是B选项的 on other crops，我觉得指代不太清楚，是指的基因改变型吗，请教大家谢过