Meaning: 'Although' means 'in spite of the fact that,' indicating a contrast between two statements.

Wrong options:

• Since: 'Since' implies a cause-and-effect relationship, which does not fit the contrasting nature of the sentence.
• Unless: 'Unless' suggests a condition that must be met, which does not align with the context of contrasting knowledge and action.
• However: 'However' is typically used to introduce a statement that contrasts with a previous one, but it does not fit grammatically at the beginning of the sentence.

Correct answer: had worked

Explanation: The phrase 'had worked' is in the past perfect tense, indicating that the action of working out how to extract graphene was not completed before another past action (the isolation in 2004). This fits grammatically and contextually as it reflects the timeline of events.

Meaning: 'Had worked' refers to the past perfect tense, indicating that no one had completed the action of figuring out extraction before the isolation occurred.

Wrong options:

• works: 'Works' is present tense and does not fit the past context of the sentence.
• working: 'Working' is a gerund and does not fit grammatically as it cannot serve as the main verb in this context.
• work: 'Work' is base form and does not convey the necessary past perfect aspect required here.

Correct answer: came

Explanation: The word 'came' is the simple past tense of 'come,' which is appropriate here as it describes a completed action in the past. This fits grammatically and contextually with the narrative of how the scientific feat occurred.

Meaning: 'Came' means to arrive at a certain point or to happen, indicating the occurrence of the scientific feat.

Wrong options:

• come: 'Come' is the base form and does not fit the past tense context of the sentence.
• has come: 'Has come' is present perfect tense, which does not align with the past narrative of the story.
• had came: 'Had came' is grammatically incorrect as 'came' is the simple past form and should not be used with 'had'.

Correct answer: necessarily

Explanation: The word 'necessarily' indicates that the experiments were not required to be linked to their day jobs, fitting grammatically and contextually as it modifies the verb 'linked' to express a lack of requirement.

Meaning: 'Necessarily' means 'as a necessary result' or 'inevitably,' indicating that the experiments were not bound to their professional responsibilities.

Wrong options:

• fully: 'Fully' suggests completeness, which does not convey the intended meaning of lack of requirement in this context.
• solely: 'Solely' implies exclusivity, which does not fit the context of optionality in their experiments.
• indirectly: 'Indirectly' suggests a non-direct relationship, which does not accurately reflect the intended meaning of the experiments not being required to be linked.