Logical reasoning PrepTest 155 · Section 1 · Question 23

This one weakens the argument by pointing out the comparison flaw we identified. If life forms that evolve "elsewhere" produce different isotope ratios than life forms on Earth, then comparing the ratios of the Martian meteorite to those on Earth's minerals doesn't make much sense, 

This one also weakens the argument by pointing out the comparison flaw we identified. If the ratios are affected by climate and environment, and if Earth and Mars have very different climates, the isotope ratio might not have the same meaning on Mars as on Earth. Therefore, comparing the ratios of the Martian meteorite to those on Earth's minerals doesn't make much sense, and certainly won't allow us to draw a sweeping conclusion about life on Mars.

Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Weaken (Bizarro)

Stimulus Summary:
Biological activity leads to a change in the ratio of two particular isotopes on Earth. A meteorite from Mars has ratios of those isotopes only found in minerals formed before life existed on Earth. So life probably never formed on Mars.

Strategy Overview:

1. Argument or facts? Always argument, so identify premises and conclusions of argument
2. Anticipate why the premises are not enough to show that the conclusion is true
   1. Causal argument? Look for answer choice that:
      1. Shows possible alternate cause
      2. Shows cause without effect
      3. Shows effect without cause
      4. Shows the cause and effect are reversed
   2. If not causal argument, anticipate ways to worsen problem with argument
3. Use anticipations to eliminate the answer choices that weaken the argument and select an answer choice that does not weaken the argument.

Answer Anticipation:
This is a Bizarro Weaken question, so the argument has to be flawed enough that four different answer choices can weaken it. And this one is very flawed indeed! There are several common fallacies committed here.

First, we should notice that this is conclusion is about the entire history of Mars, so we should think about temporal fallacies. Maybe the Martian meteorite was ejected from that planet before life emerged. Even if that's true, life still could have emerged on Mars after the meteorite was ejected. In other words, this argument is using evidence that may only tell us about a small piece of a Mars's history to make a conclusion about all of Mars's history.

There's also the issue of comparing one planet to a different one. We know that life changes the ratio of these isotopes on Earth. But do we know that it works that way on Mars? Maybe Martian life wouldn't change this isotopic ratio in the same way.

There is also a part-to-whole or sampling issue. Even ignoring the temporal and comparative issues, the meteorite may only tell us about one part of Mars — the part from which the meteorite came. Even if life hadn't formed on that part of Mars, life still could have originated elsewhere on Mars.

We'll use these anticipations to help us eliminate wrong answer choices. The correct answer choice will not weaken the argument. It could be irrelevant, or could even strengthen the argument by fixing one of these problems.

Answer Choice Explanation:
This strengthens the argument by showing that the meteorite was, at least at one time, representative of the entire planet of Mars. This fixes the sampling/part-to-whole flaw we identified. Since we're looking for anything that doesn't weaken, an answer that strengthens is correct.

Key Takeaway:
Arguments in Bizarro Weaken questions are especially bad, so don't stop at anticipating just one flaw! We identified at least three, and each helped us eliminate the wrong answer choices and select the correct one.

This weakens the argument by suggesting that surveying Earth rocks would also produce a whole lot of rocks with the no-life isotope ratio. We know Earth has life, so one random Mars rock probably isn't any more representative of Mars than a random Earth rock is of Earth.

This weakens the argument by pointing out the temporal issue we identified. It suggests that Mars could have changed in ways that make life more likely since the time this meteorite broke off from the Red Planet.