Explanation

The blank calls for words that can refer to the tendency of a bacterium to be killed by an unfamiliar virus. “Susceptibility” and “characteristic,” although not synonyms, can both be used to indicate this trait, and because it is clear that they are both referring to the same behavior, they produce sentences alike in meaning. The other choices contain a pair of synonyms—“theory” and “hypothesis”—that might at first seem tempting. But these, along with the other two choices, cannot aptly be used to characterize the typical behavior of bacteria, so they are all incorrect. Thus the correct answer is susceptibility (Choice A) and characteristic (Choice C).
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in sentence equivalence both words have to be equivalent,so if characteristic and susceptibility individually fit they are not equivalent so theory-hypothesis should be the answer.

"a (_) surprisingly, be beneficial to the colony in which the bacterium lives"
From what I understand the blank should be filled with something that has a materialistic potential to benefit the bacteria community. Both the theory and hypothesis are synonymous and TRAPS. It will benefit and that is somewhat the theory is.... so the theory refers to something else and that should be the correct answer. So susceptibility and characteristic, though not synonyms are the right answers.

Regards,
Scrat

thanks a lot

How to identify when the synonyms are traps?
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Carcass whats the trick to identify that synonyms are traps?
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Simply know them.

Improve your vocabulary or the understanding of the sentence
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Indeed a good question as synonyms are not the answer!

Well noted.
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A particular bacterium that has never encountered a particular virus will usually succumb to it, a _______ that may, surprisingly, be beneficial to the colony in which the bacterium lives.


The comma indicates continuation of the idea. Therefore, the word for the blank has something to do with succumbing to a virus.

A particular bacterium that has never encountered a particular virus will usually succumb to it, a _______ that may, (possible word for the blank - "a tendency to succumb")

The word "surprisingly" also introduces a contrast between

a _______ that may,

and

be beneficial to the colony in which the bacterium lives.

That means the word for the blank should indicate something unfavorable, such as the act of succumbing to the virus (already established because of the comma)

The best word from the choice list is susceptibility.

The only other word which makes a meaningful sentence while conveying the same meaning as a sentence that uses susceptibility for the blank is characteristic.

Hence the correct choices are susceptibility and characteristic.

An excellent SE questions that demonstrates the use of non-synonyms to produce two sentences that are identical.
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