It can be inferred from the passage that when describing bacteria as "swimming up a concentration gradient", the author means that they were behaving as if they were swimming A. Against a resistant medium that makes their swimming less efficient. B. Away from a substance to which they are normally attracted. C. Away from a substance that is normally harmful to them. D. From an area where the concentration of a repellent is weaker to an area where it is completely absent. E. From an area where the concentration of a substance is weaker to an area where it is stronger.