The correct choice in Weaken questions will provide new information that makes the conclusion less likely. Amil has provided good explanation of why choice A is correct. Even if Choice A were not there, you can see that choice B does little to weaken Scott's claim. Scott's Claim
: A(budworms declining) -> B(Tennessee Warbler declining). Note this decline is at the source (in North America). While making this statement, Scott is claiming that the population of Tennessee Warbler is declining at the source because Tennessee Warbler does not get enough food
at source. Note, that Scott does not say that the cause of decline of bud-worms is the same as the cause for decline of Tennessee Warbler.
What does choice B say?
Choice B states that the cause for decline of budworms (infection) does not "directly" impact Tennessee Warbler
However, Scott never claimed the above to be the case. He only claimed that Tennessee Warbler declined because budworm declined. Hence this new information does not weaken Scott's case. What could be another correct choice?
Any answer choice that states that the population of Tennessee Warbler is healthy in North America would be a good choice. Hence a choice that stated that the population of Tennessee Warbler in North America at the beginning of the migration period this year was no different than that last year. - would be correct.
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