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，什么意思？是个什么assumption，感觉很难理解。谢谢解答！
B的意思是, 老的品种在去年的产量不可能达到以前的产量(EX.100MILLION), 所以说减产是因为RESEARCHER不开发新品种(可能是老品种反复种会减产, 我猜)
还是一个句子的理解问题，这个句子有点费解
就是一个not +weaken吧
大概明白了：
Option B can be understood as: the last-year yields of potatoes most commonly grown in Rosinia was unlikely to reach 100 tonnes, whatever happens.
necessary grammar for this question:
We use may/might/could (not 'can') + have + past participle to say it is possible that something happened in the past.
Caution: The modifier of potatoes--'most commonly grown in Rosinia'--is necessary for B to be a key. Because the researchers would be exempted from the complaint if there was not the modifier. That is, if the general term, potatoes, had been used in B, the last-year yields could not have reached the past level even though the researchers had developed a new strain of potato which could bring about more yields under the common environmental circumstances.
In fact, B functions as precluding the other reasons for the decrease in the yields of potatoes. 不过B实在是出得太隐蔽了，太有水平了。现在对last的出题人越来越佩服了。一个正确答案能绕好几个圈说出来，还保持逻辑上的严整。太牛了。！！！！！
明白了~
题目说：Twenty years ago the Republic of Rosinia produced nearly 100 million tons of potatoes, but last year the harvest barely reached 60 million tons.
那么去年所有potatoes产量的减产，也有可能不是因为potatoes most commonly grown in Rosinia 的减产而造成的，而是因为别的品种的减产造成的；而如果说要怪科学家没有开发新品种而导致了所有土豆的减产的话，必须要有前提说去年的减产是因为传统品种减产造成的，才能以这个理由怪科学家。B正是这个意思。
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