PrepTest 131, Section 3, Question 3
Hemoglobin, a substance in human blood, transports oxygen(61 more words not shown)
Which one of the following(8 more words not shown)
- AA hemoglobin molecule(14 more words not shown)
- BThe only factor(22 more words not shown)
- CA hemoglobin molecule(30 more words not shown)
- DA hemoglobin molecule(24 more words not shown)
- EEach hemoglobin molecule(26 more words not shown)
Reading the argument
Each oxygen molecule makes the next one easier to pick up. Start with a hemoglobin molecule carrying none. After it picks up one, it becomes more effective at picking up another. After the second, it becomes more effective again; after the third, more effective still. Four is the limit.
The passage also explains the physical change: picking up oxygen changes the hemoglobin molecule's shape, opening it to receive more. We aren't given an argument with a disputed conclusion. We're given a process to understand.
What we’re looking for
For the most strongly supported answer, keep the claims as limited as the description. We know that effectiveness increases as oxygen is added, up to the capacity of four. I'll check each answer against that process and keep the one the passage supports.
Working the answers
- A
More effective doesn't necessarily mean likely to succeed. Suppose the chance of picking up another oxygen molecule increased from 5 percent to 10 percent. That would be an improvement, but another pickup would still be unlikely. These figures illustrate the distinction; the passage gives us no probability that would establish A's claim.
- B
The passage identifies a factor affecting how well hemoglobin picks up oxygen. It doesn't say this is the only factor. Perhaps something else, such as conditions associated with smoking, could also matter. We need not establish that possibility as a fact to see that the passage never rules other factors out.
- CCorrect
The molecule carrying three has gone through more of the increases in effectiveness than the molecule carrying only one. Since each additional oxygen molecule makes it more effective until the maximum of four is reached, the molecule with three is better at picking up another than the molecule with one. That comparison directly follows the process described.
- D
The passage describes repeated changes that open the molecule to receive more oxygen. It gives us no reason to think that reaching four restores the shape it had before it picked up any. A return to the original shape is an added claim, not a supported result of the description.
- E
We are told that hemoglobin transports oxygen from the lungs, so that part fits. The problem is the claim about each molecule picking up between one and four. Some molecules might not pick up any oxygen. Describing what happens when a molecule picks up oxygen doesn't establish that every molecule does so.
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