LR Bible, p. 292 - October 1999, LR2, Q20 (amphibians)

[Nikki Siclunov]

The following question arrived at our desk earlier today:

The question is about amphibians, I understand why answers B-E strengthen the argument (although I incorrectly selected D the first time around - I understand now how D could eliminate another cause for population decrease, this strengthening the conclusion) but I think it is completely bogus that A is correct - is it simply because the answers fails to mention amphibians? The whole test is about assumptions and this and that - I am just wondering if there was a different perspective or explanation for this one, as I'm starting to get frustrated with how nuanced the LR section can be at times.

Thanks!

[Nikki Siclunov]

Our response:

Thanks for the question! So you understand why B-E strengthen the argument: they either eliminate alternate causes [answer choice (D)], or else bolster the correlation between the disappearing amphibian populations and the depleted ozone layer [answer choice (C)], or else indicate that a “control group” - animals that aren’t as vulnerable to UVB as amphibians are - do not experience a similar decline [answer choice (B)]. Last, answer choice (E) provides additional evidence of a coincidence between the purported cause and effect, bolstering the conclusion.

By asking why answer choice (A) is correct, you’re essentially asking why answer choice (A) does not strengthen the argument. Well, why do you think it does? You probably believe (and correct me if I’m wrong here) that answer choice (A) eliminates the possibility that other types of radiation would be responsible for the genetic damage sustained by the amphibians, thus strengthening the argument that we should blame UVB for their declining populations. The thing is, this wasn’t the argument! The conclusion blames the depletion of the ozone layer, not UVB, for the declining population. We know as a fact that amphibians are susceptible to UVB radiation: there is no reason to strengthen this premise. And while UVB may be the proximate cause for this extinction, eliminating alternative proximate causes (such as UVA, IR, etc.) does not strengthen the conclusion. This is because each of these proximate causes would only result in the effect described if the ozone layer is actually depleted! Notice how they worded (A): “of the various types of radiation blocked by atmospheric ozone…”. So, regardless of which particular wavelength of radiation ends up killing the amphibians, we can still blame the depletion of the ozone layer for it, because the ozone was supposed to block that wavelength (but did not).