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I chose A for this question but I think it is wrong because it acts as a conclusion rather than an additional premise?

Planet 253 ---> no methane ---> no life on p 253
No methane ---> no microbes

D: No micobes ---> planet 253 ---> no methane ---> no life on planet 253

Is this correct?
 Adam Tyson
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Inserting "Planet 253" into the conditional chain is a bit confusing, theamazingrace. Your first diagram would read something like "If Planet 253, then no methane, and if no methane then no life." Your diagram of answer D reads as "If there are no microbes then it is Planet 253, and if it is Planet 253 then there is no methane, and if there is no methane then there is no life on Planet 253." That's odd, and also an incorrect interpretation of what that answer is doing. Take the planet out of the chain, because it is not a conditional element. It's just a place where these conditions occur.

The goal here is to prove the conclusion, which is that there is no life on that planet. The evidence presented is enough to allow us to infer that if there is no methane, then there can be no microbes, but that doesn't tell us anything about life. To justify a conclusion about life, we need to close the gap between "no microbes" from the premises and "no life" in the conclusion. We need an answer that says that if there are no microbes then there cannot be life, which answer D does. Here's how that might look:

Premise: It appears that there is no methane

Premise: Microbes always produce methane

Sub-conclusion: If there is in fact no methane, there are no microbes

Main conclusion: If there is no methane, there is no life

You can see how the first two premises do a good job of supporting the sub-conclusion here, but the main conclusion comes out of nowhere talking about life when we have zero evidence about life. We need a new premise between the sub-conclusion and the main conclusion that connects the absence of microbes to the absence of life. That's what we should prephrase, and what we should look for in the answer choices - a connection between microbes and life. Answer A is incorrect because it tells us nothing at all about life, and we are supposed to be proving the conclusion that there is no life.

Be careful about building conditional chains that are not based on "if this, then that" statements! Also, when faced with a Justify the Conclusion question, if the conclusion of the argument has some new, "rogue" element in it, be sure that your prephrase deals with that new thing and that you reject any answer that fails to connect to that new thing! You may be able to answer a question like this one with no diagram at all if you just take that mechanistic approach.

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