求解一道逻辑题

baizhantang

Twenty years ago the Republic of Rosinia produced nearly 100 million tons of potatoes, but last year the harvest barely reached 60 million tons. Agricultural researchers, who have failed to develop new higher-yielding strains of potatoes, are to blame for this decrease, since they have been concerned only with their own research and not with the needs of Rosinia.  prep6 3-15

Which is an assumption?

答案：Strains of potatoes most commonly grown in Rosinia could not have produced the yields last year that they once did.
为什么是这个答案？

fjatpku

题中说减产的原因是failed to develop new higher-yielding strains of potatoes
但failed to develop new strains其本身还不足以引起减产，中间还少一个链接条件
即Strains of potatoes most commonly grown could not have produced the yields last year that they once did.
有这个条件后
failed to develop new higher-yielding strains of potatoes这个条件与减产的结果就可以联系起来了

另一方面 如果negate这个assumption, 就必然有scientists failed to develop new strains之外的因素才能导致减产了

baizhantang

谢谢楼上指点