CR - GMAT Prep

D

If all the earth bacteria are coming from Mars bacteria then conclusion fells apart.
Remember this is a weaken question , so find the choice that attacks the conclusion

my ans is : D

This argument is a great case of "gmat logic"--the answer choice that ends up weakening the conclusion is pretty bizarre (albeit possible), and probably not something that most regular people would have identified as an assumption on an initial read (all bacteria on Earth could have come from Mars? Say wha?) But when we're super specific about what we've been given, that's the choice we arrive at.

CONCLUSION: If bacteria arrived on Earth from Mars, they must have died out.

WHY? (Premises)
(1) bacteria from diff planets would have major differences in protein structure
(2) no two bacteria on Earth are different enough to meet criteria in (1)

The conclusion is a CAUSAL CLAIM-- IF something happened, THEN something else must be true. Keep that in mind as we go to the choices:

(A) We don't care if bacteria actually did develop on Mars. We only care if it DIED OUT after arriving on Earth from Mars. Remember what we said about the causal claim above? If bacteria never developed on Mars (and therefore never arrived on Earth from Mars), we never fulfilled the "if" part of the statement so we have no basis to judge the "then" portion. This choice is therefore out of scope.

(B) Who cares if it was likely or not? We need to know IN THE EVENT THAT bacteria were transported, what happened after-- must it have died out or not? This choice deals with the first part of the "if" statement but give us no information about the second part. Eliminate.

(C) We don't care how the bacteria got there. Bacteria *could* have arrived via meteor, but the conclusion deals with IF it arrived, what happened to it AFTER-- (died out or not?). The method of transport is irrelevant.

(E) We don't care about other Earth bacteria, we only care about potential Martian bacteria on Earth. Eliminate.

Choice (D) plugs an assumption hole: the author assumes that because the bacteria on Earth are not as different in protein structure as bacteria from two different planets would be, that (1) the bacteria must have come from ONE planet, and that (2) the planet must be EARTH...therefore even if bacteria came from Mars, we don't have any left so it must have died out. If, however, all bacteria on Earth came from MARS (so the one planet is NOT Earth), then that would explain the similarly-structured bacteria on Earth AND weaken the argument that all Martian bacteria on Earth died out. This is our answer.