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P: Today's farmers plant only few different strains of a given crop.
P: Few generations ago there were wider diversity
C: If a disease strikes only a few strains of crop --> that would have only minor impact on food supply in the past
If a disease strikes only a few strains of crop --> it would devastate it today

We are asked to weaken the argument.

B (Correct) Affected crops can quickly be replaced from seed banks that store many strains of those crops.

I don't really like it. If disease X would strike 3 of 5 strains of crop that are planted today, then what will happen? What part of crop would die? AC "B" doesn't prevent possibility that all of those 3 strains of crop would die entirely. But even if there is a seed bank with seeds, that seeds can't compensate supply lost, because time is needed to reestablish crop population.

"B" used word "quickly" in regard to replacement of affected crops. But what does it mean? Can crops germinate in one week and be ready to harvest on second week?

But even if crops can somehow quickly be replaced, it only weakens second part of conclusion: If a disease strikes only a few strains of crop --> it would devastate it today. But there is also "If a disease strikes only a few strains of crop --> that would have only minor impact on food supply in the past" part. Did seed banks exist few generations ago? We don't know

D. Humans today have more variety in their diets than in the past, but still rely heavily on cereal crops like rice and wheat.

It looks more appealing than "B" to me. Weakens both parts of conclusion by introducing something that differentiates past from present. If humans have more variety in their diets than in the past then they have an access to a wider variety of products. Therefore they have more options for substitution.

Can you help me to figure out why "B" is better than "D" please?

Ok, so first why D is wrong: it deals with people's diet variety today; this has nothing to do with the food supply (as in, the total amount of food in supply).

B is definitely correct; unfortunately, in your reasoning against B, you completely contradicted what B actually said. You made the assumption that:

"...even if there is a seed bank with seeds, that seeds can't compensate supply lost, because time is needed to reestablish crop population."

If you carefully read B, it stipulates that the affected crops CAN be quickly replaced from seed banks. Further, it replaces them with other strains of crops unaffected by disease, thereby supplying the diversity which the argument suggests has been lost.

Gotta read those answer choices carefully remember that strengthen/weaken questions ask "which of the following IF TRUE"; you must assume that the answer choices are true; if you make assumptions against their inherent truthfulness, you are not following the question stem's instructions

For the purpose of simplification, let's rephrase the stimulus.
Support: Farmers are planting fewer crops compared to past generations.
Conclusion: A disease, which in the past, would've only made a minor dent in the food supply, will devastate it today.

Now, the stem asks for an answer which would, if true, most weaken the argument that planting fewer crops will devastate today's food supply.

(B) would sufficiently weaken the argument because it gives a reason why the stated conclusion can be false. If affected crops can be quickly replaced, then the conclusion that the food supply will be devastated is weakened.

(D), on the other hand, irrelevantly speaks of "variety." Who knows what "more variety" means? (Maybe in the past we had a choice of 10 crops and now we have a choice of only 7. "Variety" means nothing without data.) Moreover, (D) mentions heavy reliance on two cereal crops. If there was a heavy reliance on cereal crops, the support that "planting fewer crops would devastate the food supply" would strengthen the argument because of a "heavy reliance on cereal crops." Remember, a "heavy reliance on cereal crops" means that not only are the farmers planting fewer crops but, rather, we are relying on only a portion of those few crops. Thus, the conclusion that a disease would hurt real bad is strengthened by (D).
Thus, (D) is wrong for two reasons, while (B) creates a wedge between the support and the conclusion.

Thank you guys, now I see why "B" is better than "D".

D is an interesting wrong answer. First part before comma says that "Humans today have more variety in their diets than in the past." As zhenders rightly pointed out "more variety" has nothing to do with total amount food in supply. But it still could have weakened conclusion by itself.

If human diet is diversified then damage to any particular type of food doesn't hurt so much; The more choice people have the less they depend on any particular type of food. So maybe people today rely less on crops in their diet?

But after comma comes "but still rely heavily on cereal crops like rice and wheat" part. It eliminates possibility mentioned above!

Walrus wrote: I don't really like it. If disease X would strike 3 of 5 strains of crop that are planted today, then what will happen? What part of crop would die? AC "B" doesn't prevent possibility that all of those 3 strains of crop would die entirely. But even if there is a seed bank with seeds, that seeds can't compensate supply lost, because time is needed to reestablish crop population.
<snip>
But even if crops can somehow quickly be replaced, it only weakens second part of conclusion: If a disease strikes only a few strains of crop --> it would devastate it today. But there is also "If a disease strikes only a few strains of crop --> that would have only minor impact on food supply in the past" part. Did seed banks exist few generations ago? We don't know

One thing to remember about weaken questions is that the correct answer choice does not have to completely destroy the argument, or make the conclusion impossible. It only has to make the conclusion less likely. Similarly, if there are two different elements to a conclusion, weakening one of them does the job! A weakener is not require to weaken all independent parts of a conclusion to be a valid weakener.

But it's important that you realize that there are not actually two parts to the conclusion here. The phrase "and that would have had only minor impact on the food supply in the past" is not a conclusion! It is merely further describing the hypothetical disease. In other words, this hypothetical disease has two characteristics: 1) strikes only a few strains of crops and 2) would have had a minor impact in the past. The conclusion is simply that that disease, the one with those two characteristics, would devastate food supply today.