Meaning: 'Strongest' refers to having the highest strength or durability.

Wrong options:

• thinnest: 'Thinnest' is the superlative form of 'thin,' which does not fit the context of describing strength.
• aggressive: 'Aggressive' describes a behavior and does not relate to the material's strength.
• confused: 'Confused' is an adjective that indicates a lack of clarity or understanding, which is irrelevant to the description of spider silk.
• regard: 'Regard' is a noun that does not fit grammatically or contextually in this sentence.

Correct answer: aggressive

Explanation: The term 'aggressive' describes the behavior of spiders, indicating that they are hostile towards each other. This fits contextually as it explains why they cannot live close together, maintaining the logical flow of the passage.

Meaning: 'Aggressive' means ready or likely to attack or confront; characterized by hostility.

Wrong options:

• confused: 'Confused' describes a state of uncertainty, which does not apply to the behavior of spiders in this context.
• failed: 'Failed' is a verb that does not describe a characteristic of spiders and is irrelevant here.
• strongest: 'Strongest' refers to strength, which does not pertain to the social behavior of spiders.
• thinnest: 'Thinnest' describes width and does not relate to aggression.

Correct answer: failed

Explanation: The word 'failed' is the past tense of 'fail,' indicating that attempts to produce silk in laboratories were unsuccessful. This fits grammatically and contextually, as it describes the outcome of those attempts.

Meaning: 'Failed' means not achieving the desired result or outcome.

Wrong options:

• aggressive: 'Aggressive' describes behavior and does not fit as a verb indicating an outcome.
• qualities: 'Qualities' is a noun that does not relate to the action of attempts.
• strongest: 'Strongest' is an adjective that does not convey an outcome and is irrelevant here.
• confused: 'Confused' is an adjective that does not apply to the context of attempts.

Correct answer: qualities

Explanation: The term 'qualities' refers to the characteristics or attributes of spider silk. This fits grammatically as a plural noun that complements the context of discussing the properties of silk produced by goats.

Meaning: 'Qualities' means the features or characteristics that define something.

Wrong options:

• failed: 'Failed' is a verb that does not fit as a noun describing characteristics.
• aggressive: 'Aggressive' describes behavior and does not relate to the attributes of silk.
• strongest: 'Strongest' is an adjective that does not fit as a noun in this context.
• thinnest: 'Thinnest' is a superlative adjective that does not describe the characteristics of silk.