PrepTest 135, Section 2, Question 23
Certain bacteria that produce hydrogen sulfide as a(68 more words not shown)
Which one of the following(8 more words not shown)
- AA dense colony(26 more words not shown)
- BThe hydrogen sulfide(16 more words not shown)
- CMost organisms, if(18 more words not shown)
- DThe bacteria can(26 more words not shown)
- EIf any colony(28 more words not shown)
Reading the argument
The bacteria's waste does two useful jobs: it protects them and helps feed them.
Oxygen would kill these bacteria on direct contact. Their hydrogen sulfide reacts with that oxygen and removes it before it can do harm. The hydrogen sulfide also tends to kill nearby organisms, creating a food source. A dense colony can therefore maintain an environment in which it keeps thriving.
What if the colony is in a Petri dish and someone throws it away? We can imagine other threats, but this question asks what the given information supports. 'Can continue to thrive indefinitely' doesn't mean nothing could ever destroy the bacteria. It describes the ongoing environment the colony can create through the two effects we've been told about.
What we’re looking for
Look for an answer that carries those effects forward without adding a new mechanism, turning this route into the only route, or extending the claim to every colony.
Working the answers
- ACorrect
A dense colony can keep producing enough hydrogen sulfide to maintain both benefits: protection from oxygen and the killing of nearby organisms that supply food. That connects the ongoing production to the ongoing favorable environment described in the final sentence. It doesn't claim that the colony is immune to every conceivable outside threat.
- B
We know how hydrogen sulfide protects these bacteria: it reacts with oxygen. We also know it kills other organisms. We aren't told that oxygen removal is what kills those other organisms. This answer takes the stated mechanism of one effect and assigns it to the other.
- C
The killed organisms provide a food source, but the passage doesn't quantify how many kinds or individuals are edible to the bacteria. Some could supply food while others did not. That is enough for a food source to exist without establishing C's claim about most organisms.
- D
This makes the described environment the only one in which the bacteria can keep thriving. The passage gives us a successful route, not proof that no other environment could support them. D also requires removing all oxygen, while the stated benefit is preventing oxygen from harming the bacteria.
- E
The passage concerns a dense colony of these particular bacteria. A sparse colony might fail to produce enough hydrogen sulfide to establish the same protection and food supply. Producing some of the chemical doesn't automatically guarantee both benefits for any colony.
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