Parts of the Bacteria
The argument claims that the use of a flagellum to swim requires many parts. Therefore, if something did not have those parts, it would gain no survival advantage from a flagellum. There’s a serious gap here!
Evidence: ~MP â†’ ~UFS
Conclusion: ~MP â†’ ~SA
Gap: ~UFS â†’ ~SA
(key: MP = many parts, UFS = use a flagellum to swim, SA = survival advantage)
Back into English, taking the contrapositive of the gap, "In order to gain a survival advantage from using a flagellum, something would have to use it to swim." This is best expressed in answer choice (B).
(A) does not bridge a gap in the argument but rather seems to be merely a restatement of the information within the conclusion.
(B) fills the gap and is the assumption of the argument.
(C) is not necessary to the argument. Even if it were the case that some parts of the flagellum were not vital, the conclusion could still be true.
(D) misses the topic at hand. It’s not about whether these ancestors had the parts needed to use the flagellum, it’s about a hypothetical situation dealing strictly with those ancestors that did not have all of the parts needed to use a flagellum to swim.
(E) isn’t needed for the argument to stand. Even if they could swim by some other means, the parts of the flagellum might still not offer any survival advantages.