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This is the question where you have to weaken the argument that heat trapped in the atmosphere correlated with a high presence of carbon dioxide.

I'm having trouble understanding why (B) is the answer. Are we supposed to assume that because the oceans stayed liquid, the level of greenhouse gasses was high, and still is today? So, by choosing (B) we weaken the argument b/c it states that the level was higher beforehand. I need some explanation please.

The conclusion of this argument is that the levels of carbon dioxide back then (3 billion years ago) we're probably quite high. Why? Because the oceans remained liquid, showing that the temperature was rather high, even though the sun was not as strong as it used to be. It's a fact, we're told, that the only way to bring about such a temperature back then would have been through a high level of greenhouse gases. . . So, there must have been a lot of CO2.

What's the gap? Why does it have to be CO2? Couldn't it have been methane?

(B) plays on this assumption by suggesting that there used to be more methane (though it's a bit confusing since it seems to be discussing how much methane is in the atmosphere now.

(A) simply rules out another reason that might explain the heat -- if anything, this strengthens the argument.
(C) is irrelevant. The effect mentioned is not a possible alternate explanation.
(D) is out of scope as it relates the sun's strength with life forms.
(E) is also out of scope. We do not know what role increased radiation plays in this argument; furthermore, we don't know when this increase has reached a level that may or may not effect the earth's temperature as the increase is only limited to some time over the last 3 billion years.

I hope that helps.

Thanks a lot. Both your responses helped.