The flagellum, which bacteria use to swim, requires many parts before it can propel a bacterium at all. Therefore, an evolutionary ancestor of bacteria that had only a few of these parts would gain no survival advantage from them.

Question: Which one of the following is an assumption on which the argument depends?
Options:
A. Any of bacteria’s evolutionary ancestors that had only a few of the parts of the flagellum would be at a disadvantage relative to similar organisms that had none of these parts.
B. For parts now incorporated into the flagellum to have aided an organism’s survival, they would have had to help it swim.
C. All parts of the flagellum are vital to each of its functions.
D. No evolutionary ancestor of bacteria had only a few of the parts of the flagellum.
E. Any of bacteria’s evolutionary ancestors that lacked a flagellum also lacked the capacity to swim.
Answer: "B"