Meaning: 'Infectious' refers to bacteria that can cause disease and spread from one person to another.

Wrong options:

• zealous: 'Zealous' means showing great energy or enthusiasm, which does not relate to the type of bacteria being discussed.
• vulnerable: 'Vulnerable' means susceptible to harm, but it does not accurately describe the bacteria that antibiotics are designed to eliminate.
• capricious: 'Capricious' means given to sudden changes in mood or behavior, which is irrelevant in the context of bacteria and antibiotics.

Correct answer: mutating

Explanation: The word 'mutating' is the correct choice as it describes the process by which bacteria change to resist antibiotics. This fits both grammatically and contextually within the sentence.

Meaning: 'Mutating' means undergoing a change, particularly in genetic structure, which allows bacteria to adapt and survive against antibiotics.

Wrong options:

• retaining: 'Retaining' means keeping or holding onto something, which does not convey the idea of bacteria changing to resist drugs.
• commuting: 'Commuting' refers to traveling back and forth, which is unrelated to the context of bacterial resistance.
• upset: 'Upset' means to disturb or displace, which does not fit the context of bacteria adapting to antibiotics.

Correct answer: different

Explanation: The word 'different' is appropriate as it indicates that the bacteria may become vulnerable to another type of drug. This fits grammatically and contextually within the sentence.

Meaning: 'Different' means not the same as another or others, highlighting the potential for bacteria to be susceptible to an alternative antibiotic.

Wrong options:

• referenced: 'Referenced' means mentioned or cited, which does not fit the context of discussing types of drugs.
• interfaced: 'Interfaced' refers to connecting or interacting, which is not relevant in this context.
• dissimulating: 'Dissimulating' means concealing or disguising one's thoughts or feelings, which does not apply to the discussion of drug susceptibility.

Correct answer: knockout

Explanation: The word 'knockout' fits well as it conveys the idea of delivering a decisive blow to the bacteria, aligning with the context of the sentence about combating antibiotic resistance.

Meaning: 'Knockout' refers to a powerful blow that incapacitates or defeats, particularly in a competitive context.

Wrong options:

• ragout: 'Ragout' is a type of stew, which is irrelevant in the context of discussing antibiotics and bacteria.
• layoff: 'Layoff' refers to dismissing employees from their jobs, which does not relate to the context of fighting bacteria.
• sought: 'Sought' is the past tense of seek, meaning to look for, which does not fit the context of delivering a blow.

Correct answer: susceptible

Explanation: The word 'susceptible' is the correct choice as it describes bacteria that have become more vulnerable to a second drug after mutation. This fits grammatically and contextually.

Meaning: 'Susceptible' means likely to be influenced or harmed by a particular thing, indicating the bacteria's increased vulnerability.

Wrong options:

• sustainable: 'Sustainable' refers to the ability to be maintained or continued, which does not apply to the context of bacterial resistance.
• liable: 'Liable' means likely to do or experience something, but it does not convey the specific vulnerability of bacteria to drugs.
• dependent: 'Dependent' means relying on something else, which does not accurately describe the relationship between bacteria and antibiotic resistance.