I wanted to ask why D would be wrong. I didn't choose C because it talks about the presence rather than the concentration, which is the main focus. I didn't choose E because of the same reason paho mentioned. It didn't say whether it was attractant or a repellent. Like you mentioned, it would have been a much better and 100% accurate answer if it included "attractant".
However, that is exactly what D does except it's the other way around. In the first paragraph, the passage says "They are attracted to materials they need and are repelled by harmful substances.
". So we can infer that just as the bacteria move towards the attractant, they move away from repellent.
The question asks "the author means they were behaving as if
they were swimming". If the bacteria is moving away from higher to lower concentrations of repellent (weaker to absence is still higher concentration to lower concentration), it would behave as if
they were swimming from lower to higher concentrations of attractant with a few tumbles (which is actually true). I thought this was a reasonable inference from the given information and E seemed to be missing critical information, which is whether the substance was an attractant or a repellent, that gave me enough reason to eliminate it.
Could you help me find my mistake and why E is accurate, let alone the best answer?
The only way I could see why E is an answer is if I were to look at the quoted phrase "swimming up a concentration gradient" by itself without looking at the context in the passage.
Swimming up a concentration gradient = moving from lower concentration of any substance to higher concentration
However, because the question was asking "as if"
, I don't see why E would be wrong. Also, the statement from E, "from an area where the concentration of a substance is weaker to an area where it is stronger", would be wrong as I could argue that bacteria would not behave like the bacteria in the context if the substance happens to be a repellent, not an attractant.
I'm not sure when to look at the context and when to look solely at the quoted phrase.