LSAT Question Explanation
PT 105, Section 1, Question 19
StrengthenArgument structure
It's false that life began in the oceans and didn't exist on Earth until half a billion years ago.
There are 1.2 billion year old rocks in North America with Carbon 14, which is released by some life forms when they die.
Explanation
This is a Strengthen "except" question, so four of the answer choices will strengthen the conclusion and one correct answer choice will either weaken the conclusion or do nothing.
This is a flawed stimulus. Sure, land based life forms could be one way the carbon 14 got into the rocks, but it could have gotten there some other way. Also the author seems to assume that those rocks weren't in the ocean sometime in the last 1.2 billion years.
I would be looking for strengthening answer choices that fix those flaws or strengthen the conclusion in some other way, and then I would cross them out because this is an "except" question.
Answer choices
This definitely strengthens, it tells us that there were land based life forms prior to 500 million years ago.
This shows that part of the view our author rejects (that life started in the oceans) would have been very difficult. So it strengthens that maybe life started elsewhere.
This one is trying to trick us, because it leads by saying that some of the rocks with Carbon 14 were underwater, which makes it seem like ocean-based rather than land-based life forms may have left the Carbon 14 there. But then (C) says that some of the rocks were never underwater! The existence of these never-submerged rocks eliminates an alternate explanation for the Carbon 14 and strengthens the conclusion that there was life on land over 500 million years ago.
This introduces an alternate explanation as to how the Carbon 14 got into the rocks, weakening the idea that it was left there by land-based life forms, which weakens that life existed over 500 million years ago.
This strengthens that the age of the rock was measured correctly, which strengthens the whole argument.