PrepTest 147, Section 1, Question 23
Zoologist: Plants preferentially absorb heavy nitrogen from rainwater.(71 more words not shown)
Which one of the following,(8 more words not shown)
- APlants can also(11 more words not shown)
- BThe rate at(19 more words not shown)
- CThe same number(15 more words not shown)
- DBone samples from(17 more words not shown)
- EThe level of(25 more words not shown)
Reading the argument
Maybe the cave bears ate unusually nitrogen-rich plants. Imagine an Ice Age berry that gave them very high nitrogen levels even though they ate no meat. Matching the levels found in meat-eating bears would then be less revealing about their diet than the zoologist thinks.
The food-chain explanation does make the argument plausible. Plants absorb heavy nitrogen, plant-eaters concentrate it, and meat-eaters have still higher concentrations. But our hypothetical berry shows why we shouldn't immediately treat a high level as conclusive evidence of meat eating.
What we’re looking for
To strengthen the argument, direct evidence that the cave bears ate meat would help. Another possibility would be evidence that eating meat was the only way for them to reach the observed nitrogen level. That would connect the measurement to the conclusion that they weren't exclusively plant-eaters.
Working the answers
- A
Additional sources of nitrogen for plants don't tell us whether the cave bears ate meat. We already know plants absorb heavy nitrogen. Learning that it can come from somewhere besides rainwater doesn't establish what diet produced the bears' measured levels.
- B
This concerns inferring the rate of accumulation in the blood of Ice Age herbivores from their bones. Our question concerns what a measured nitrogen level tells us about the cave bears' diet. We haven't established that the cave bears were herbivores, and a way to infer an accumulation rate in known herbivores doesn't supply the comparison needed for this conclusion.
- C
The numbers of samples needn't match exactly for a useful comparison. Five samples from one group and six from another, for example, would not automatically make the evidence unusable. Saying the counts are equal doesn't explain why the measured levels show that the cave bears ate meat.
- DCorrect
This exposes a different issue: the original comparison used the cave bears' bones and the modern bears' blood. Are those measurements comparable? D tells us that, in the modern meat-fed bears, the bone and blood levels are the same. We can now connect the matching samples: the cave-bear bone level matches the modern blood level, and that modern blood level matches the modern bone level. So the cave-bear bones also match the bones of bears known to eat meat. That makes the dietary comparison stronger. It doesn't resolve every possible alternative, such as the hypothetical nitrogen-rich plant, but it repairs an important mismatch in the evidence.
- E
This says bears already known to eat meat share the same bone-nitrogen level. It doesn't tell us that only meat-eaters can have that level; plant-eaters might match it too. It also doesn't connect that shared bone level with the blood level used in the original comparison. We therefore can't use E alone to move from the matching bone-and-blood samples to the cave bears' diet. D supplies that missing comparison directly.
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