Meaning: 'Through which' means by means of or using, indicating the method of observation that was unavailable to astronomers.

Wrong options:

• across which: 'Across which' suggests a spatial relationship that does not fit the context of observing the sky, as it implies movement rather than the means of observation.
• from that: 'From that' implies a source or origin, which does not logically connect to the idea of using optical aids for observation.
• in which: 'In which' suggests a location or situation, which does not accurately convey the method of observation intended in this context.

Correct answer: during

Explanation: The word 'during' appropriately indicates a time period in which something occurs, fitting grammatically and contextually as it refers to the closing years of the 16th century when telescopes began to appear.

Meaning: 'During' means throughout the duration of a specific time period, indicating that the events happened within that timeframe.

Wrong options:

• when: 'When' is a conjunction that introduces a specific point in time, which does not convey the ongoing nature of the period being described.
• by the time: 'By the time' suggests a deadline or specific moment, which does not fit the continuous aspect of the closing years being discussed.
• for: 'For' indicates a duration but does not specify the time frame as effectively as 'during' does.

Correct answer: the

Explanation: The article 'the' is used to specify a particular noun, in this case, 'Italian astronomer Galileo Galilei,' making it grammatically correct and contextually appropriate as it identifies him as a notable figure in astronomy.

Meaning: 'The' is a definite article used to refer to a specific person or thing that is already known to the reader.

Wrong options:

• a: 'A' is an indefinite article that suggests any one of a group, which does not apply here as Galileo is a specific individual.
• some: 'Some' implies an unspecified quantity, which does not fit the context of identifying a specific astronomer.
• since: 'Since' is a preposition indicating time, which does not function as an article and does not fit grammatically.

Correct answer: Since

Explanation: The word 'Since' indicates a starting point in time and is used correctly here to show that from that moment onward, the telescope has been essential for astronomers, fitting both grammatically and contextually.

Meaning: 'Since' means from a particular time in the past until now, indicating the ongoing relevance of the telescope in astronomy.

Wrong options:

• After: 'After' indicates a sequence of events rather than a starting point, which does not fit the context of ongoing relevance.
• For: 'For' suggests a duration of time but does not indicate a starting point, making it less suitable in this context.
• From: 'From' indicates a starting point but lacks the temporal continuity that 'Since' provides.

Correct answer: tool

Explanation: The word 'tool' is appropriate as it refers to an instrument used by astronomers for their work, fitting grammatically and contextually as it describes the telescope's role in astronomy.

Meaning: 'Tool' refers to an instrument or device used to carry out a particular function, in this case, aiding in astronomical observations.

Wrong options:

• capability: 'Capability' refers to the ability to do something, which does not fit the context of a physical instrument like a telescope.
• means: 'Means' can refer to a method or instrument but is less specific than 'tool' in this context.
• resource: 'Resource' is a broader term that can refer to anything that can be used for support, making it less precise than 'tool' in describing the telescope.