About 3 billion years ago, the Sun was only 80 percent as luminous as it is currently. Such conditions today would re...

Nicolette on September 25, 2014

Why B?

I do not understand why mentioning methane in the argument leads to it be a considerable part of the correct answer. It is compared to carbon dioxide because it is a greenhouse gas. It is not necessarily included with carbon dioxide as part of the necessary components of trapping heat within the atmosphere in the argument, or at least this is not made clear.

1 Reply

Mehran on September 26, 2014

This is a cause and effect argument. The observed effect that the author is trying to explain is why water rather than ice filled the Earth's oceans about 3 billion years ago when the Sun was only 80 percent as luminous as it is currently.

The author's proposed cause? The level of carbon dioxide in the atmosphere was significantly higher then than it is today.

Why? Heat is trapped within Earth's atmosphere through "greenhouse gas" so only if the level of greenhouses gases were higher 3 billion years ago than it is today would Earth have retained enough heat to keep the oceans from freezing.

We are being asked to weaken this cause and effect argument. We know there are three ways to weaken a cause and effect argument: (1) alternate cause, (2) cause without effect and (3) effect without cause.

Answer choice (B) states "Geological studies indicate that there is much less methane in Earth's atmosphere today than there was 3 billion years ago."

The stimulus tells us that methane is a "greenhouse gas," so (B) would weaken this argument by showing alternate cause (i.e. it was not carbon dioxide that was significantly higher but rather methane).

Hope this helps! Please let us know if you have any other questions.