PrepTest 101

Incorrect. The argument assumes that bacteria isn't on the surface of wooden cutting boards and thus can't contaminate food, so it doesn't make any assumptions about how bacteria can or can't be removed from the surface. If anything, this statement cuts against the argument.

Incorrect. The argument doesn't "rank" wooden and plastic cutting boards, so it's not assuming anything about superiority, let alone that one respect is the only respect in which wooden boards are better than plastic ones.

Incorrect. This answer is too strong to be necessary. The argument still holds together even if uncontaminated food can end up contaminated by being rubbed in garbage before placed on a cutting board!

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Plastic cutting boards - Bacteria stay on surface for several days
Wooden cutting boards - Bacteria is absorbed, leaving the surface
Conclusion - Wooden cutting boards can be cleaned by wiping them off instead of wiping them to stop food contamination

Answer Anticipation:
In Strengthen with Necessary Premise questions, we generally want to start with the conclusion to see if there are any new concepts raised that would need to be connected to premises. Here, however, we see something interesting when looking at the conclusion—it's focused on wooden cutting boards, whereas the premises talk about both wooden and plastic cutting boards. While it does say something about plastic boards, that's an aside and not a central feature of the conclusion. As such, we should focus on wooden cutting boards.

The conclusion states that wooden cutting boards just need to be wiped off, not washed, to prevent contamination of food cut on it. Why does the argument believe that's the case? Because bacteria penetrate wooden cutting boards almost immediately, leaving the surface free of contamination.

That may mean that there's no bacteria on the surface right after use, but there's no indication that bacteria inside of the wooden cutting boards can't eventually contaminate food. Maybe they eventually make their way back to the surface, or maybe they reproduce until there's so much bacteria it teems out to the surface. Let's find an answer ruling out these possibilities—that once bacteria is inside of a wooden cutting board, it doesn't get back to the surface.

Answer Explanation:
This answer matches our anticipation. If the bacteria that penetrate into the wooden cutting boards reemerge later, then just wiping them down won't prevent contamination, killing the argument. This answer is therefore a necessary premise for the argument, so it's correct.

Key Takeaway:
Notice that the flaw in the stimulus has a temporal element to it—it assumes what is true immediately after use continues to be true. Whenever the LSAT is talking about a timeline of any kind—even with language like "immediately"—see if it's projecting or jumping to another time. If it is, you've likely found a gap in the logic.

Incorrect. Since the stimulus argues that washing isn't necessary to prevent surface contamination, what is true of the effect of washing on below-surface bacteria is out of scope, and thus this answer isn't a necessary premise.