Logical reasoning PrepTest 133 · Section 2 · Question 18

Incorrect. The conclusion that the partial flagellum provided no survival advantage is an absolute statement—it didn't help the bacteria to survive. It's not a relative statement comparing it to other organisms, so this answer is out of scope.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
The bacterial flagellum needs many parts to swim, so an ancestor of bacteria that didn't have all of these parts was useless.

Answer Anticipation:
The argument establishes that a flagellum needs many parts in order to allow bacteria to swim. Therefore, if the flagellum doesn't have all of those parts, it's useless.

However, the argument doesn't conclude that a bacterial ancestor without all of the parts of the flagellum couldn't swim using the flagellum—it concludes that the partial flagellum wasn't useful at all. There could be some other survival advantage of that partial flagellum, but the argument assumes there isn't.

Since the argument assumes that there isn't some other survival benefit of a partial flagellum, the correct answer will either swing big by establishing that, or it'll be more limited and rule out a single other survival advantage that the partial flagellum might provide. After all, in order to establish that something doesn't provide any survival advantage, each survival advantage that it could provide must be ruled out.

Answer Explanation:
This answer essentially rules out all other potential survival benefits the partial flagellum could provide by saying that swimming is the only potential benefit of it. If there are other potential benefits, then the partial flagellum might actually provide a survival benefit, killing the argument. This is therefore the correct answer.

Key Takeaway:
Starting your analysis with the conclusion and working backwards to see how the premises fall short of establishing it is one way to approach your analysis of a flawed argument. Another is to start with the premises and determine what they do allow you to conclude, and then see how the actual conclusion differs from this. Here, the premises would allow the conclusion to be drawn that the partial flagellum wouldn't allow these ancestral bacteria to swim. The conclusion was about survival advantages generally. Seeing that jump would lead straight to the correct answer.

Incorrect. The argument doesn't require all of the parts of the flagellum are necessary for swimming, just enough that the "few" present in the ancestral bacteria wasn't enough for it to swim.

Incorrect. The argument's conclusion is speculative/qualified—it draws a conclusion about what would be true if such an organism existed. As such, it doesn't matter if such an organism actually existed.

Incorrect. While the argument requires that the partial flagellum couldn't allow the organism to swim, it doesn't require that the ancestors didn't have some other means of swimming. This answer would be correct if it stated that the hypothetical ancestors with partial flagella couldn't swim, not that the partial flagellum didn't provide a survival advantage.