PrepTest 151

Incorrect. This answer strengthens the argument by removing the supposed cause of this behavior and showing the behavior disappears.

Incorrect. The argument states that the bacteria move based on the amount of energy produced in certain areas, and it's been established that shade is the best for energy production.

Incorrect. By ruling out an alternative explanation (temperature driving this behavior), this answer strengthens the argument.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Weaken

Stimulus Summary:
Bacteria with chlorophyll move to an area of test tubes that is colored with light that is best for chlorophyll energy production, so that color of light and the energy production associated with it must be how the bacteria know where that light is.

Answer Anticipation:
This argument features a correlation/causation flaw (or, to look at it another way, it ignores alternative explanations for why the bacteria move to the red light), but it doesn't feature as clear language on that front as many other questions we've seen (which is why it's question #22). Even the conclusion obscures that causality! However, the heart of this argument is that the red light and associated energy production is causing the bacteria to move only into certain areas—a causal assumption that leads to a conclusion that goes out of its way to avoid using causal language. As such, we should look for our traditional correct answers to Weaken questions featuring this jump:

(1) Identifying alternative causes (why the bacteria would move to the red light outside of energy production)
(2) Counterexamples (bacteria not moving to areas that maximize energy production; bacteria moving to areas that don't maximize energy production)
(3) Reversed causality (which doesn't make much sense here, since the light preceded the movement)

Answer Explanation:
By showing a situation where the cause is present (an area that would create as much energy), but the effect is absent (bacteria moving there), the argument calls into question the connection between the bacteria's movement and the amount of energy they produce in those areas.

Key Takeaway:
Weaken (and Strengthen) questions have a strong association with correlation/causation flaws, so you should be on the lookout for that flaw even if there's no specific causal language in the conclusion. The LSAT has recently been featuring arguments with causal assumptions rather than causal conclusions, so any language that implies a causal relationship should get you to start thinking about this flaw.

Incorrect. These bacteria presumably do still need energy, so this answer is out of scope without information about what they use to generate energy and how different colors of light interact with those sources.