Meaning: 'Its discovery' refers to the event of penicillin being found, emphasizing the importance of that moment in medical history.

Wrong options:

• been discovered: This option is a passive perfect participle phrase that does not fit the grammatical structure required here, as it lacks a subject and does not convey the intended meaning of a specific event.
• was discovered: While this is a grammatically correct passive construction, it does not fit contextually as it suggests a completed action rather than the significance of the event itself.
• of discovery: This phrase is a prepositional phrase that does not function as a subject or object in the sentence, making it grammatically incorrect in this context.

Correct answer: deadly

Explanation: The word 'deadly' fits contextually as it describes the nature of infections before penicillin, indicating that they could lead to death. It also matches grammatically as an adjective modifying 'infections.'

Meaning: 'Deadly' means capable of causing death, highlighting the severity of infections prior to the advent of penicillin.

Wrong options:

• divergent: This term means differing or deviating, which does not relate to the context of infections and their severity.
• contemporary: This word refers to something existing at the same time, which is irrelevant to the description of infections' fatality.
• unsightly: While this means unpleasant to look at, it does not convey the seriousness of infections, making it an inappropriate choice.

Correct answer: transformed

Explanation: The verb 'transformed' is appropriate as it indicates a significant change in healthcare due to penicillin. It fits grammatically as the past tense verb aligns with the subject 'it' referring to penicillin's impact.

Meaning: 'Transformed' means to change something significantly, indicating that penicillin greatly altered the landscape of healthcare.

Wrong options:

• intitiated: This verb means to begin or start something, which does not capture the extent of change that penicillin brought to healthcare.
• diagnosed: This verb refers to identifying a disease, which is not relevant to the context of penicillin's impact on healthcare.
• contributed: While this verb suggests a positive addition, it lacks the strong implication of change that 'transformed' conveys.

Correct answer: fighting

Explanation: The gerund 'fighting' fits grammatically as it serves as a noun in the context of the sentence, indicating the action of combating infections. It aligns contextually with the ongoing role of penicillin in healthcare.

Meaning: 'Fighting' refers to the act of combating or resisting bacterial infections, emphasizing the active role of penicillin in treatment.

Wrong options:

• injecting: This verb implies the act of administering a substance, which does not accurately describe the role of penicillin in treating infections.
• disabling: This term means to make something unable to function, which does not appropriately describe the action taken against infections.
• perceiving: This verb means to become aware of something, which is unrelated to the context of treating infections.