请教LSAT6-III-15

joywzy

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，我觉得B，D皆可。

joywzy

我明白了。

D的双重否定把我搞糊涂了。是削弱。

回家过年

D是双重否定，说20年内的收成波动没有什么不正常（正常）。实际是削弱了“由于科学家光忙呼自己的事造成减产”的结论。
B强调了一下前提的有效性（很隐蔽哦），大部分土豆产量都没达到以前的水平。是假设答案比较隐蔽的一种。
这题很好，很典型！谢谢