Meaning: 'By' in this context means 'through the means of' or 'as a result of,' indicating that the discovery was unintentional.

Wrong options:

• at: 'At' indicates a specific location or time, which does not fit the context of how the discovery occurred.
• for: 'For' suggests purpose or benefit, which does not align with the accidental nature of the discovery.
• in: 'In' typically refers to a location or condition, which does not convey the means of occurrence as required here.

Correct answer: another

Explanation: 'Another' indicates an additional quantity or instance, which is appropriate here as it refers to the 500 years that passed before the eureka moment reached Europe.

Meaning: 'Another' means 'one more' or 'an additional one,' emphasizing the time gap before the technology spread to Europe.

Wrong options:

• few: 'Few' implies a small number, which does not accurately describe the significant time span of 500 years.
• less: 'Less' is used for uncountable nouns and does not fit with the countable time period of years.
• further: 'Further' suggests a greater distance or extent, which does not apply to the context of time elapsed.

Correct answer: with

Explanation: The preposition 'with' indicates accompaniment or association, fitting well as it describes how the spread of technology occurred alongside the movement of the Celtic tribes.

Meaning: 'With' means 'together with' or 'in association with,' highlighting the connection between the spread of technology and the Celtic tribes.

Wrong options:

• along: 'Along' suggests a linear movement or direction, which does not accurately convey the relationship between the spread of technology and the tribes.
• without: 'Without' indicates absence, which contradicts the idea that the spread of technology was linked to the Celtic tribes.
• for: 'For' implies purpose or benefit, which does not fit the context of describing the relationship between two subjects.

Correct answer: made

Explanation: The verb 'made' indicates creation or transformation, fitting grammatically and contextually as it describes how iron contributed to improving life.

Meaning: 'Made' means 'to cause to become' or 'to create,' indicating that iron technology significantly improved daily life.

Wrong options:

• caused: 'Caused' implies a trigger for an event, which does not fit the context of describing the role of iron in improving life.
• became: 'Became' suggests a change of state, which does not accurately describe the function of iron in this context.
• begot: 'Begot' is an archaic term meaning to produce or generate, which is less commonly used and does not fit the modern context of the sentence.

Correct answer: toiling

Explanation: The gerund 'toiling' indicates continuous effort or labor, fitting well as it describes the ongoing action of working the soil with tools.

Meaning: 'Toiling' means 'working hard' or 'laboring,' emphasizing the effort involved in farming during that time.

Wrong options:

• having: 'Having' indicates possession, which does not convey the action of working the soil.
• burdening: 'Burdening' implies causing a load or stress, which does not accurately describe the act of farming.
• treading: 'Treading' suggests walking or stepping, which does not fit the context of actively working the soil.

Correct answer: to exploit

Explanation: The infinitive form 'to exploit' indicates purpose or intention, fitting grammatically as it describes the farmers' aim in using iron tools.

Meaning: 'To exploit' means 'to make full use of' or 'to utilize effectively,' indicating the farmers' intention to maximize their agricultural efforts.

Wrong options:

• exploit: The base form 'exploit' does not fit grammatically in this context as it lacks the necessary infinitive form.
• exploits: 'Exploits' is a third-person singular form, which does not match the intended subject of the sentence.
• exploited: 'Exploited' is the past tense form, which does not convey the intended future action of the farmers.