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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"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