PrepTest 131

Incorrect. First, the stimulus specifically notes that higher greenhouse gas levels are necessary for the Earth to retain enough heat to keep the oceans liquid. Second, were that not the case, this answer eliminates an alternative means by which the Earth could have been heated, which would strengthen the argument that it was greenhouse gases.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Weaken

Stimulus Summary:
3 billion years ago, the Sun wasn't hot enough to melt Earth's oceans, but they were water
not Oceans frozen (i.e, oceans water) → Greenhouse gas levels higher 3 billion years ago
Therefore: Likely that carbon dioxide levels were higher 3 billion years ago

Answer Anticipation:
While this stimulus isn't the most straightforward application of a conditional we've ever seen, it does have that structure. The stimulus raises a conditional premise ("Only if") that was triggered by an earlier statement—though it is a bit hard to see. That conditional statement's sufficient condition is that Earth retained enough heat to keep the oceans from freezing, and we know that the oceans were water rather than ice in the time period in question. The conditional, therefore, lets us conclude that greenhouse gas levels were higher 3 billion years ago.

However, it concludes that carbon dioxide levels were higher 3 billion years ago. While carbon dioxide is established as a greenhouse gas, it's not the only one—in fact, the argument itself mentions methane as another greenhouse gas. If the levels of other greenhouse gases were high enough to warm the Earth enough to stop the oceans from melting, then carbon dioxide levels wouldn't need to have been higher. Any answer that raises this possibility will weaken this argument.

Answer Explanation:
This answer highlights that methane—another greenhouse gas—was present in much higher levels in the atmosphere 3 billion years ago. If that's the case, then that greenhouse gas could account for the increased temperature that stopped the oceans from freezing, and carbon dioxide levels wouldn't need to be higher. This answer proposes an alternative explanation for how the oceans stayed liquid, so it undermines the argument.

Key Takeaway:
When the argument itself raises an alternative possible explanation, you should pay attention! The correct answer will frequently bring that alternative up to address the question stem.

Incorrect. Similar to (A), this answer both eliminates an alternative means by which the oceans could stay liquid, and tries to work around the conditional that required greenhouse gas levels to be higher 3 billion years ago.

Incorrect. Lifeform complexity is out of scope of the argument.

Incorrect. This answer choice might explain why higher levels of greenhouse gases aren't required to keep the Earth's oceans liquid today, but it doesn't impact an argument about 3 billion years ago.