This argument basically says that plants preferentially absorb heavy nitrogen from rainwater, creating a cascade. Herbivores will have higher levels of heavy nitrogen in their tissues, and consequently, so will meat eating animals. The argument then compares the level of heavy nitrogen in the bones of ice-age bears with the level of heavy nitrogen in the blood of modern day meat eating bears. The levels were the same. The argument then concluded that ice-age bears ate meat.
The issue with this conclusion is that it relies on comparing two different things. Levels of heavy nitrogen may very well be different in bones vs blood.
Answer choice D fixes this issue by stating that present day bears have the same level of heavy nitrogen in bones as they do blood, which allows this comparison to make sense. There is no longer the huge doubt that perhaps bone level should be much higher or lower than blood level.
Answer choice B doesn't work because we are trying to support the argument that this comparison between the ice-age bear's bones and modern day bear's blood can indicate anything about the diet of the ice-age bear. Telling us the speed at which the level of heavy nitrogen reached the blood from bones of ice-age bears doesn't help this argument. We need only to establish that the comparison is valid. Not the mechanisms or underlying facts about heavy nitrogen absorption, accumulation, etc.
I hope this helps. Please let me know if you have any other questions!
tomgbeanDecember 14, 2019
Thanks!
RaviJanuary 14, 2020
@tomgbean, let us know if you have any other questions!
