Meaning: 'Unavoidably' means that something cannot be avoided or prevented.

Wrong options:

• extraordinary: 'Extraordinary' is an adjective that describes something remarkable, which does not fit as it does not modify the verb 'heading.'
• unfortunately: 'Unfortunately' is an adverb that expresses regret, but it does not appropriately describe the manner of the asteroid's approach.
• alarmingly: 'Alarmingly' is an adverb that indicates something is causing alarm, but it does not fit the context of the asteroid's trajectory.
• abundant: 'Abundant' is an adjective that means plentiful, which does not relate to the direction or manner of the asteroid's movement.
• including: 'Including' is a preposition that introduces examples, which does not fit grammatically in this context.
• significant: 'Significant' is an adjective that describes importance, which does not modify the verb 'heading' appropriately.
• realization: 'Realization' is a noun that refers to becoming aware of something, which does not fit the grammatical structure needed here.

Correct answer: alarmingly

Explanation: The word 'alarmingly' is an adverb that describes the degree to which the situation seemed threatening. It fits grammatically as it modifies the adjective 'threatening' and aligns with the context of the asteroid's potential impact.

Meaning: 'Alarmingly' means in a way that causes alarm or concern.

Wrong options:

• extraordinary: 'Extraordinary' is an adjective that does not fit as it cannot modify 'seemed' or 'threatening' appropriately.
• unavoidably: 'Unavoidably' is an adverb that indicates certainty but does not convey the sense of concern needed in this context.
• unfortunately: 'Unfortunately' is an adverb expressing regret, which does not fit the context of describing the threat level.
• abundant: 'Abundant' is an adjective that means plentiful, which does not relate to the degree of threat.
• including: 'Including' is a preposition that introduces examples and does not fit grammatically in this context.
• significant: 'Significant' is an adjective that describes importance but does not convey the alarming nature of the threat.
• realization: 'Realization' is a noun that does not fit grammatically or contextually in this sentence.

Correct answer: significant

Explanation: The word 'significant' is an adjective that describes the level of risk posed by the asteroid. It fits grammatically as it modifies the noun 'risk' and aligns with the context of discussing the potential danger.

Meaning: 'Significant' means important or considerable in amount or effect.

Wrong options:

• extraordinary: 'Extraordinary' is an adjective that describes something remarkable, which does not fit the context of describing a risk level.
• unavoidably: 'Unavoidably' is an adverb that does not modify 'risk' and does not fit grammatically in this context.
• unfortunately: 'Unfortunately' is an adverb expressing regret, which does not describe the nature of the risk.
• abundant: 'Abundant' is an adjective that means plentiful, which does not relate to the level of risk.
• including: 'Including' is a preposition that introduces examples and does not fit grammatically in this context.
• alarmingly: 'Alarmingly' is an adverb that describes the manner of something but does not fit as a descriptor for 'risk.'
• realization: 'Realization' is a noun that does not fit grammatically or contextually in this sentence.

Correct answer: abundant

Explanation: The word 'abundant' is an adjective that describes the quantity of evidence available. It fits grammatically as it modifies the noun 'evidence' and aligns with the context of discussing the frequency of past asteroid strikes.

Meaning: 'Abundant' means existing or occurring in large amounts.

Wrong options:

• extraordinary: 'Extraordinary' is an adjective that describes something remarkable, which does not fit the context of describing the quantity of evidence.
• unavoidably: 'Unavoidably' is an adverb that does not modify 'evidence' and does not fit grammatically in this context.
• unfortunately: 'Unfortunately' is an adverb expressing regret, which does not describe the quantity of evidence.
• including: 'Including' is a preposition that introduces examples and does not fit grammatically in this context.
• significant: 'Significant' is an adjective that describes importance but does not convey the quantity of evidence.
• alarmingly: 'Alarmingly' is an adverb that describes the manner of something but does not fit as a descriptor for 'evidence.'
• realization: 'Realization' is a noun that does not fit grammatically or contextually in this sentence.

Correct answer: extraordinary

Explanation: The word 'extraordinary' is an adjective that describes the remarkable nature of the crater. It fits grammatically as it modifies the noun 'one' and aligns with the context of discussing significant past events.

Meaning: 'Extraordinary' means very unusual or remarkable.

Wrong options:

• unfortunately: 'Unfortunately' is an adverb expressing regret, which does not fit as a descriptor for 'one.'
• alarmingly: 'Alarmingly' is an adverb that describes the manner of something but does not fit as a descriptor for 'one.'
• unavoidably: 'Unavoidably' is an adverb that does not modify 'one' and does not fit grammatically in this context.
• abundant: 'Abundant' is an adjective that means plentiful, which does not fit as a descriptor for 'one.'
• including: 'Including' is a preposition that introduces examples and does not fit grammatically in this context.
• significant: 'Significant' is an adjective that describes importance but does not convey the remarkable nature of the crater.
• realization: 'Realization' is a noun that does not fit grammatically or contextually in this sentence.