I understand the appeal of (B); when I first read it, it appealed to me to. Then I read (D) and realized why (B) is wrong. Let me explain.
On a weaken question, you are looking for the answer choice that, if added to the argument structure, would weaken or undermine the conclusion. To do this properly, let's be sure we are totally clear on what the relevant portion of the argument in the passage is:
Premise: altered PS could crowd out non-altered PS
Premise: and the altered PS would only change the pathogen's propensity to cause frost damage (and not in any other way alter the original pathogen), so
Conclusion: altering PS is safe.
Answer choice (B) is irrelevant, because it goes to how the altered pathogen is derived. The answer says "the altered bacteria is derived from . . ." I know it says that it is derived from a strain that can cause crop damage, but there is nothing in answer choice (B) that establishes the altered pathogen itself will cause crop damage. Thus, answer choice (B) does not weaken the argument, and is incorrect.
Think about it this way. Let's say Virus X is very dangerous, but scientists can splice one little part of Virus X (which is itself not dangerous) and add it to Bacteria C to make Bacteria C less harmful. Can you see how there is no evidence that Bacteria C would thereby be as dangerous as Virus X? It will all depend on how Virus X is used to modify Bacteria C. There is nothing necessarily or inherently dangerous about this process.
Now consider answer choice (D), which says "Often genes whose presence is responsible for one harmful characteristic in order to prevent other harmful characteristics." OK, well this does weaken the argument outlined above, because if PS is altered to remove the gene responsible for frostbite (one harmful characteristic), and that then means PS cannot prevent OTHER harmful characteristics, then the argument (that altering PS in this way is safe) falls apart.
Hope this helps! Please let us know if you have any additional questions.
anselcarpenterNovember 7, 2017
I'm confused by the word 'Often'.
Doesn't that word imply that it is not necessarily the case here that removing the frostbite gene would cause more problems, similar to how answer B is wrong because it is not necessarily the case that the derived bacteria will be harmful?
