Generally speaking, if the same crop is sown in a field for several successive years, growth in the later years is po...

Naz January 29, 2014

This is a Paradox question and the words that should tip you off that are: "most helps to explain."

The stimulus states that if a crop is sown in a field several times in a row, growth in the later years is poorer because nitrogen in the soil is depleted. Then the stimulus mentions alfalfa and the fact that even though it is a nitrogen-fixing plant (i.e. it increases the amount of nitrogen in the soil), for some reason it acts as the non-nitrogen-fixing plants it just mentioned, in that it too still grows less well in later years than earlier ones if planted in the same field consistently.

The question stem then asks which answer choice helps explain the paradox of why the non-nitrogen-fixing plants and alfalfa (a nitrogen-fixing plant) have this similar quality of growing poorer if planted in the same field year after year.

Answer choice (C) is the correct answer because it is a possible explanation to the paradox in the stimulus. If it's true that alfalfa produces a substance that accumulates in the soil that is toxic to it, then the reason why it grows poorer year after year is explained despite the fact that its nitrogen supply has not been depleted.

Hope that helped! Please let us know if you have any other questions.

Batman November 11, 2015

Thanks a lot!!

Crystal December 20, 2016

Why D is not correct?

Mehran December 20, 2016

(D) is not a possible explanation to the discrepancy that alfalfa, a nitrogen-fixing plant which increases the amount of nitrogen in the soil, nevertheless grows less well in later years and it does in earlier years.

(D) tells us that alfalfa increases nitrogen in the soil in which it grows only if a certain type of soil bacteria is present in the soil.

"Only if" introduces the necessary condition so (D) would be diagrammed as follows:

AIN ==> SBP
not SBP ==> not AIN

The stimulus, however, has indicated that alfalfa increases the amount of nitrogen in the soil but nevertheless grows less well. All (D) establishes is that if alfalfa increases nitrogen in the soil, then we know a certain type of soil bacteria is present.

But this does not resolve the discrepancy that when alfalfa increases the amount of nitrogen in the soil, it still grows less well.

Notice (C) resolves this discrepancy by pointing out something else, i.e. alfalfa is producing substances that accumulate in the soil and that are toxic to alfalfa.

Hope this helps! Please let us know if you have any other questions.