被gwd逻辑搞昏了，求教6套21题

最后阶段冲刺了，好像一有时间限制练套题就没法把逻辑每题看懂，是不是应该在仔细分析大量题的基础上锻炼短时间内做逻辑题能力啊？
下附怎么都弄不明白的第6套21题，求各位大侠赐教

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
为什么回是D呢？不是已经说清楚比较的是每亩的吗？那尝试和没尝试的种的谷物总亩数怎会有影响？