Meaning: 'Persistance' refers to the quality of continuing to do something despite difficulties or opposition.

Wrong options:

• determination: 'Determination' implies a firmness of purpose but does not specifically convey the idea of ongoing effort over time, making it less suitable in this context.
• steadiness: 'Steadiness' suggests stability but lacks the connotation of ongoing effort or struggle, which is central to the idea of enduring continuance.
• diligence: 'Diligence' refers to careful and persistent work or effort, but it does not directly relate to the concept of enduring over time as 'persistance' does.

Correct answer: stage

Explanation: The term 'stage' is appropriate here as it refers to a specific point or period in a process, which fits the context of scientists giving up on projects at various points in their development.

Meaning: 'Stage' means a particular point or period in a process of development.

Wrong options:

• period: 'Period' is a broader term that could refer to any length of time, but it does not convey the idea of a specific point in a process as effectively as 'stage'.
• step: 'Step' implies a single action or movement in a sequence, which does not capture the idea of a phase in a project’s timeline.
• gap: 'Gap' suggests a break or interruption, which does not fit the context of scientists continuing or discontinuing their work.

Correct answer: purify

Explanation: The verb 'purify' fits grammatically and contextually as it describes the action Fleming was attempting to perform on penicillin, aligning with the scientific context of isolating a substance.

Meaning: 'Purify' means to remove impurities or unwanted elements from a substance.

Wrong options:

• isolate: 'Isolate' means to separate something from others, which is related but does not specifically convey the action of making something pure.
• stabilize: 'Stabilize' refers to making something stable or steady, which does not align with the intent of refining or cleaning a substance.
• process: 'Process' is a noun that refers to a series of actions, not an action itself, making it grammatically incorrect in this context.

Correct answer: reasons

Explanation: The word 'reasons' is suitable as it refers to the explanations or justifications for Fleming's decision to stop working on penicillin, fitting the context of providing rationale.

Meaning: 'Reasons' are explanations or justifications for why something is done or why a decision is made.

Wrong options:

• causes: 'Causes' refer to factors that bring about an effect, which does not fit the context of explaining a decision.
• findings: 'Findings' refer to results or conclusions from research, which does not align with the context of explaining why he discontinued his work.
• investigations: 'Investigations' imply a process of inquiry or examination, which does not match the context of providing reasons for a decision.

Correct answer: unstable

Explanation: The adjective 'unstable' fits grammatically and contextually as it describes the nature of penicillin as perceived by Fleming, indicating that it was not reliable or consistent.

Meaning: 'Unstable' means not steady or firm; likely to change or fail.

Wrong options:

• undesirable: 'Undesirable' means not wanted or sought after, which does not specifically convey the idea of inconsistency or unreliability.
• unreliable: 'Unreliable' suggests a lack of dependability but does not capture the idea of being subject to change as effectively as 'unstable'.
• untenable: 'Untenable' refers to something that cannot be maintained or defended, which does not fit the context of describing the nature of penicillin.

Correct answer: process

Explanation: The noun 'process' is appropriate as it refers to a series of actions taken to achieve a particular end, which aligns with the context of discovering a method to purify penicillin.

Meaning: 'Process' refers to a series of actions or steps taken to achieve a particular end.

Wrong options:

• project: 'Project' refers to a planned undertaking, which does not fit the context of a method or series of actions for purification.
• program: 'Program' typically refers to a planned series of events or activities, which does not align with the context of a method for purification.
• plan: 'Plan' suggests a detailed proposal for doing something, which does not convey the ongoing nature of actions taken to achieve purification.