Clarification

We are told that this specific bacteria produces hydrogen sulfide (HS) as a waste product that reacts with oxygen and removes it, preventing the bacteria from being harmed. We are also told that the HS kills other organisms in the area, which provides the bacteria with a source of food. Thus, a dense colony of this type of bacteria creates an environment for itself that allows it to "continue to thrive indefinitely." Answer choice (A) states: "A dense colony of the bacteria can indefinitely continue to produce enough HS to kill other organisms in the area and to prevent oxygen from harming the bacteria." Answer choice (A) is essentially restating the last sentence of the passage: "As a result, a dense colony of these bacteria produces for itself an environment in which it can continue to thrive indefinitely." Hope that clears thing up! Please let us know if you have any other questions.

Thank you. What does "it" refer to following the word "removing"? Is it the oxygen?

Yes that is referring to Oxygen.

Why is D incorrect here? How is this different from A in that you would know to choose A? I was stuck between the two here.