Question ID:PT51 S3 Q18 Passage: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. Stem:Which one of the following is an assumption on which the argument depends? Correct Answer Choice:BChoice 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. Choice B:For parts now incorporated into the flagellum to have aided an organism's survival, they would have had to help it swim. Choice C:All parts of the flagellum are vital to each of its functions. Choice D:No evolutionary ancestor of bacteria had only a few of the parts of the flagellum. Choice E:Any of bacteria's evolutionary ancestors that lacked a flagellum also lacked the capacity to swim.