20 yrs ago's yield: 100 m tons
last yrs' yield: 60 m tons
Agricultural researchers are to blame for this decrease (failure to develop new higher yielding strains)
The strains are not high yielding (What the strains are high yielding but some other external factors interrupted)
(B) is correct!
(A) Any current attempts by agricultural researchers to develop higher-yielding potato strains are futile.Not all attempts are futile. Futile in what sense, perhaps they were able to discover something else despite failure to create a high yield strain... Whether futile or not futile, it has no bearing on the conclusion that they are to blame for the reason stated by the author
(C) Agricultural researchers often find concrete solutions to practical problems when investigating seemingly unrelated questions.Apply assumption negation. Agricultural researchers always find concrete solutions to practical problems. Does this cause the argument to fall apart? They are still to blame if the strains of potatoes are not of high-yield. Still holds!
(D) Wide fluctuations in the size of the potato crop over a twenty-year period are not unusual. Potato Size - is irrelevant to potatoes yield in volume
(E) Agricultural research in Rosinia is funded by government grants government grants is no where hinted in the argument. so this needs not be assumed.
Impossible is nothing to God.