Meaning: 'Could have expected' means that it was possible for the owners to anticipate or foresee the robot's operational limits in the past.

Wrong options:

• will be expecting: This phrase indicates a future expectation, which does not fit the past context of the sentence.
• should expect: This implies a recommendation or obligation, which is not suitable for describing past expectations.
• had to expect: This suggests a necessity or obligation in the past, which does not align with the more flexible notion of possibility conveyed by 'could have expected.'

Correct answer: leaving

Explanation: The word 'leaving' is used correctly here as it indicates the result of the previous clause, showing the outcome of the extended run times. It fits grammatically and contextually, as it describes the impression created by the new information about robot longevity.

Meaning: 'Leaving' means to cause or result in a particular impression or state, in this case, the impression that humans and robots might retire together.

Wrong options:

• taking: This does not fit the context as it implies an action that does not relate to creating an impression.
• showing: While it could fit, it does not convey the same sense of resulting impression as 'leaving' does in this context.
• making: This implies creation but does not fit the context of resulting in an impression.

Correct answer: troubling

Explanation: The word 'troubling' fits well as it conveys a sense of concern or worry, which aligns with the context of hidden issues in robot behavior. It is grammatically correct and contextually appropriate, indicating a negative pattern that needs attention.

Meaning: 'Troubling' means causing worry or concern, reflecting the underlying issues that are not immediately apparent.

Wrong options:

• tedious: This implies something boring or monotonous, which does not capture the seriousness of the issues described.
• annoying: While it suggests irritation, it lacks the depth of concern conveyed by 'troubling.'
• irritating: Similar to 'annoying,' it does not adequately express the gravity of the situation as 'troubling' does.

Correct answer: name but a few

Explanation: The phrase 'name but a few' is idiomatic and fits grammatically as it introduces examples of the issues causing robot stoppages. It is contextually appropriate as it suggests that the mentioned problems are just a small selection of the overall issues.

Meaning: 'Name but a few' means to mention only a small number of examples from a larger group, indicating that there are more issues not listed.

Wrong options:

• offer an explanation: This does not fit the context as it suggests providing a rationale rather than listing examples.
• be an exception: This implies something that does not follow a rule, which does not relate to the context of listing issues.
• be a case in point: This suggests a specific example rather than a broader list, which does not align with the intent of the sentence.

Correct answer: for

Explanation: The preposition 'for' is correctly used here to indicate the basis or foundation of optimism. It fits grammatically and contextually, as it connects the lack of basis directly to the concept of optimism.

Meaning: 'For' in this context means 'in relation to' or 'regarding,' indicating that there is little reason to feel optimistic.

Wrong options:

• of: This does not fit as it suggests possession rather than a basis for something.
• on: This implies a focus or reliance, which does not convey the intended meaning of basis in this context.
• with: This suggests accompaniment or association, which does not accurately reflect the relationship between basis and optimism.