Meaning: The phrase 'were met' means that the theories received a certain type of response or reaction, specifically skepticism and awe, at that time.

Wrong options:

• have met: 'Have met' is present perfect tense, which does not fit the historical context of the initial reception being discussed.
• are met: 'Are met' is present tense, which is inappropriate as it suggests ongoing reception rather than a past event.
• met: 'Met' is a simple past form but lacks the auxiliary verb 'were' that indicates the passive voice needed for the context.

Correct answer: gained

Explanation: The verb 'gained' fits well in the context as it indicates that over time, Einstein's theories acquired substantial empirical support, aligning with the progression described in the sentence.

Meaning: 'Gained' means to obtain or acquire something, in this case, substantial empirical support for the theories.

Wrong options:

• revised: 'Revised' implies making changes or corrections, which does not accurately describe the process of acquiring support for the theories.
• surpassed: 'Surpassed' suggests exceeding something, which does not fit the context of gaining support; it implies a comparison rather than an acquisition.
• occupied: 'Occupied' means to take up space or time, which is irrelevant in this context as it does not relate to the support for the theories.

Correct answer: once

Explanation: The word 'once' indicates a time in the past when radical ideas were rigorously tested, fitting grammatically and contextually within the sentence structure.

Meaning: 'Once' refers to a specific time in the past, suggesting that at that time, radical ideas were tested and validated.

Wrong options:

• whether: 'Whether' introduces a conditional clause, which does not fit the context of discussing a past event.
• where: 'Where' indicates a location, which is not relevant to the temporal context of the sentence.
• yet: 'Yet' implies a contrast or continuation, which does not align with the intended meaning of a past occurrence.

Correct answer: unravel

Explanation: The verb 'unravel' is appropriate as it conveys the idea of solving or making sense of complex mysteries, fitting well with the context of human intellect's capabilities.

Meaning: 'Unravel' means to solve or clarify something complicated, in this case, the mysteries of the universe.

Wrong options:

• rewrite: 'Rewrite' suggests changing the text or narrative, which does not fit the context of understanding or solving mysteries.
• liberate: 'Liberate' means to set free, which does not accurately describe the action of understanding or solving mysteries.
• trust: 'Trust' implies reliance or confidence, which does not relate to the act of discovering or understanding mysteries.