Q8 - Generally speaking, if the same crop

[wj097]

I debated between C & D for the following reason.

(C): While C does explain why alfafa is not productive yr after yr, aren't sure if plants in general, as mentioned in the stimulus, also pertains to this category of plants, as C just says Certain types of plants. I acknowledge that regardless, those plants in general would not be productive for other reason i.e., lack of nitrogen. So maybe doesn't have to

(D): the word "increase" seemed problematic since this does not show that if no bacteria then decrease (not same as "not increase"). Are there issue with this part that we don't know whether the bacteria is there or not??

Thx

[demetri.blaisdell]

Thanks for posting, wj097. This is a pretty typical explain a result question. First step is to figure out the unexpected result. Here, I have:

In general, crops get less and less productive because Nitrogen is used up.

But alfalfa puts nitrogen in to the soil but it gets less and less productive as well.

We're looking for something that will kill the alfalfa. We get that in (C). The alfalfa is putting out a poison that later kills it. The other plants ("certain plants such as alfalfa") don't matter. Even if that includes some of the nitrogen producing plants, that just provides another way for them to die. It doesn't matter if they are also hurt by toxins. We just need the alfalfa to get less and less productive.

The wrong answers:

(A) is out of scope. It's about planting crops with other crops. But the stimulus only compares planting one type of crop to planting another type of crop (alfalfa).

(B) tells us how alfalfa does it. But we don't care how it puts the nitrogen in the soil. We need a way for it to get less and less productive.

(D) looks tempting. It tells us that alfalfa only puts the nitrogen back in if a certain bacteria is present. But the problem is we already know that alfalfa is getting less and less productive (and we can't change this). There are a few problems: 1) how do we know when and if the bacteria is present? 2) if it's not there, does the nitrogen decrease or just stay the same? Basically, we would need to know a lot more about the bacteria to determine if this was the reason that alfalfa is getting less and less productive.

(E) is a bit like (D). We don't know when/how often there are walnut trees near the alfalfa. So this can't be the explanation for the general phenomenon (alfalfa plants being less and less productive over time).

I hope this helps. Let me know if you still have any questions.

Demetri