I failed to diagram the conclusion as methan --> life. Diagramming it would've shortened the time. I thought "Not all..." means "Some living beings have the ability to biologically produce methane." Since some living beings biologically produce methane. That is enough for methane detection method to be the most reliable.. But now that I know to diagram the conclusion. I can see that Not all living beings have the ability to produce methane, means there are some living beings that do not produce methane. Then I can imagine that a planet has non-methane producing living beings and we won't know it has living things because we used the methane detection method as the most reliable way.

5/5 But I took really long to answer them. lol

(Thinking out loud for my benefit) I got this question wrong because I misread the question stem(I picked D, the one that strenghtens not weakens). I felt that the way this question is phrased made it easy to misread the stem. My thought process was: the conclusion is ad is not effective, the support is that weather you watch or not the same result (same treatment, same result). So when I read D, oh! the same two groups, same treatment, same result. There you go! (WRONG)! After noting that it is the "weaken" question I should've first thought "I should find the choice that proves that the ad is in fact effective."