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.（B）

(E) Agricultural research in Rosinia is funded by government grants.

这道题答案为什么是B 我觉得A好像也不错

恳求各位大侠指点~~

A显然是错的，不是农业研究者提高产量的尝试失败，是农业研究者根本就没有研究如何去提高产量，而是做他们自己的研究去了。看文章最后一句话有关键词concerned only with

B 选项是不是说低产量的问题完全出自于品种而非其他原因：如气候或农民。
倘若低产量完全由于品种不良，则可归罪于产品研发。