A.I. Explanation:

Correct answer: rely

Explanation: The word 'rely' fits grammatically as it is a verb that indicates dependence, aligning with the context of depending on electric light. It is used correctly in the present tense, matching the overall tense of the sentence.

Meaning: 'Rely' means to depend on someone or something for support or help.

Wrong options:

• trust: While 'trust' implies confidence, it does not convey the same sense of dependence as 'rely' does in this context.
• deposit: 'Deposit' refers to placing something down or putting money in a bank, which does not fit the context of dependence on light.
• base: 'Base' suggests a foundation or starting point, which does not align with the idea of depending on electric light.

Correct answer: to see

Explanation: The phrase 'to see' is an infinitive form that indicates purpose, fitting perfectly in the context of enabling visibility at night. It follows the preposition 'on' correctly, maintaining grammatical structure.

Meaning: 'To see' means to perceive with the eyes, indicating the purpose of using electric light.

Wrong options:

• seeing: 'Seeing' is a gerund and does not fit grammatically after 'enable us' as it requires an infinitive.
• for seeing: 'For seeing' is awkward and does not fit the grammatical structure needed here, as it suggests a reason rather than purpose.
• to be seen: 'To be seen' implies visibility to others, which changes the intended meaning of the sentence.

Correct answer: source of

Explanation: The phrase 'source of' is grammatically correct as it indicates the origin of something, in this case, lighting. It fits contextually as it describes the nature of lighting being discussed.

Meaning: 'Source of' refers to the origin or provider of something, indicating that electric light is a means of illumination.

Wrong options:

• means of: 'Means of' is less specific and does not convey the idea of origin as effectively as 'source of.'
• path of: 'Path of' suggests a route or direction, which does not apply to the context of lighting.
• resource of: 'Resource of' implies a supply, which is not as precise as 'source of' in this context.

Correct answer: used

Explanation: The word 'used' is the past participle form that fits grammatically in the passive voice construction of the sentence, indicating how electric light bulbs function. It aligns with the context of describing the bulbs.

Meaning: 'Used' means that something is employed or utilized, indicating the function of the light bulbs.

Wrong options:

• that used: 'That used' is incorrect as it does not fit the passive construction required here.
• is used: 'Is used' would imply a present continuous action, which does not match the context of the sentence.
• which used: 'Which used' is grammatically incorrect as it does not fit the structure needed for passive voice.

Correct answer: switched on

Explanation: The phrase 'switched on' is a phrasal verb that indicates the action of activating the light, fitting grammatically and contextually as it describes the process of using electric light.

Meaning: 'Switched on' means to activate or turn on a device, in this case, the electric light.

Wrong options:

• tuned off: 'Tuned off' is incorrect as it suggests deactivation, which contradicts the intended meaning of the sentence.
• stepped up: 'Stepped up' implies an increase or escalation, which does not relate to the action of turning on a light.
• tuned up: 'Tuned up' generally refers to adjusting something for better performance, which does not apply to the context of activating light.