15. 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.
Which one of the following is an assumption on which the argument depends?
(A) Any current attempts by agricultural researchers to develop higher-yielding potato strains are futile.
(B) Strains of potatoes most commonly grown in Rosinia could not have produced the yields last year that they once did.
(C) Agricultural researchers often find concrete solutions to practical problems when investigating seemingly unrelated questions.
(D) Wide fluctuations in the size of the potato crop over a twenty-year period are not unusual.
(E) Agricultural research in Rosinia is funded by government grants.
答案是B。为什么？题目又是什么意思？请各位XDJM赐教！
对B取非: Strains of potatoes most commonly grown in Rosinia could have produced the yields last year than they once did. 说明去年土豆产量下降是别的原因造成的， 错不在strains of potatoes, 自然错也就不在researcher, 与原文结论相反。
