Meaning: 'How' is used to inquire about the manner or method of something, in this case, the stabilization of birth and death rates.

Wrong options:

• when: The word 'when' is a temporal adverb that refers to time, which does not fit the context of explaining the process of stabilization.
• thus: 'Thus' is an adverb that indicates a conclusion or result, which does not appropriately introduce the explanation of the process.
• rest: 'Rest' is a noun that refers to remaining parts, which does not fit grammatically or contextually in this sentence.
• stare: 'Stare' is a verb that means to look fixedly, which is irrelevant to the context of demographic changes.
• too: 'Too' is an adverb that indicates excessiveness, which does not fit the context of explaining how rates stabilize.

Correct answer: thus

Explanation: The word 'thus' is used as a conjunctive adverb that indicates a result or consequence. It fits contextually as it connects the idea of development to the outcome of discarding the previous theory.

Meaning: 'Thus' means 'as a result' or 'therefore,' indicating a logical connection between the development of a nation and the discarding of the theory.

Wrong options:

• when: 'When' is a temporal adverb that does not convey the causal relationship needed in this context.
• how: 'How' is an interrogative adverb that does not fit as it does not indicate a result or conclusion.
• rest: 'Rest' is a noun that does not fit grammatically or contextually in this sentence.
• stare: 'Stare' is a verb that is irrelevant to the context of demographic changes and does not fit.
• too: 'Too' is an adverb indicating excessiveness, which does not logically connect the ideas presented.

Correct answer: too

Explanation: The word 'too' is an adverb that indicates an excessive degree. It fits grammatically and contextually as it describes the theory of overpopulation in a negative light, suggesting that the world would become excessively populated.

Meaning: 'Too' means 'excessively' or 'more than is desirable,' indicating a negative connotation regarding population growth.

Wrong options:

• when: 'When' is a temporal adverb that does not fit the context of describing the state of overpopulation.
• how: 'How' is an interrogative adverb that does not describe the state of being overpopulated.
• thus: 'Thus' is a conjunctive adverb that indicates a result, which does not fit in describing the state of overpopulation.
• rest: 'Rest' is a noun that does not fit grammatically or contextually in this sentence.
• stare: 'Stare' is a verb that is irrelevant to the context of demographic changes and does not fit.