A.I. Explanation:

Correct answer: uneconomic

Explanation: The term 'uneconomic' fits grammatically as it describes the state of being not profitable or not worth the cost, which aligns with the context of discussing fuel sources that may become difficult to obtain. It maintains the sentence's structure and meaning, indicating a potential future issue with fuel sources.

Meaning: 'Uneconomic' means not profitable or not worth the expense involved in obtaining something.

Wrong options:

• rebate: A 'rebate' is a return of part of a payment, which does not fit the context of describing fuel sources.
• renew: The word 'renew' is a verb that means to make new again, which does not appropriately describe the state of fuel sources in this context.
• suspect: 'Suspect' is a verb meaning to have doubts about something, which does not fit grammatically or contextually in this sentence.
• fossil: 'Fossil' is an adjective that describes fuels derived from ancient organic matter, which does not fit the context of describing the economic viability of fuel sources.
• constitute: 'Constitute' is a verb meaning to make up or form something, which does not relate to the economic aspect of fuel sources.

Correct answer: constitute

Explanation: The verb 'constitute' fits grammatically as it indicates that renewable energy sources make up a certain percentage of electricity usage. It aligns with the context of quantifying the contribution of renewable energy to the overall electricity supply.

Meaning: 'Constitute' means to make up or form a part of something.

Wrong options:

• rebate: 'Rebate' is a noun that refers to a return of part of a payment, which does not fit the context of describing the percentage of electricity.
• renew: 'Renew' is a verb that means to make something new again, which does not fit the grammatical structure needed here.
• suspect: 'Suspect' is a verb that means to doubt or question something, which does not fit in the context of describing a percentage of electricity.
• fossil: 'Fossil' is an adjective that describes certain types of fuels, which does not fit the context of quantifying energy sources.
• uneconomic: 'Uneconomic' is an adjective that describes something not profitable, which does not fit the context of stating a percentage.

Correct answer: fossil

Explanation: The word 'fossil' fits grammatically as it describes a type of fuel, specifically fossil fuels, which contrasts with renewable energy sources. This contextual distinction is crucial for understanding the environmental impact of different energy types.

Meaning: 'Fossil' refers to fuels derived from ancient organic matter, such as coal, oil, and natural gas.

Wrong options:

• rebate: 'Rebate' is a noun that does not relate to types of fuels and does not fit the context of the sentence.
• uneconomic: 'Uneconomic' is an adjective that describes a lack of profitability, which does not fit as a descriptor for fuel types.
• constitute: 'Constitute' is a verb that means to make up or form something, which does not fit as a descriptor for fuel types.
• renew: 'Renew' is a verb that means to make something new again, which does not fit the context of describing fuel types.
• suspect: 'Suspect' is a verb that means to doubt or question, which does not fit in the context of describing fuel types.