A.I. Explanation:

Correct answer: slip

Explanation: The verb 'slip' fits grammatically as it describes the action of microscopic pollutants moving past the body's defenses. It aligns with the context of discussing how pollutants can evade protection mechanisms.

Meaning: 'Slip' means to move smoothly and quietly, often without being noticed.

Wrong options:

• contact: The noun 'contact' does not fit grammatically as it does not describe an action that pollutants can take in this context.
• flashing: The gerund 'flashing' implies a sudden or bright display, which does not relate to the subtle movement of pollutants.
• penetrating: The verb 'penetrating' is already used in the next blank, making it redundant here and it does not fit the action of evading defenses.
• breakthrough: The noun 'breakthrough' refers to a significant advance or discovery, which does not describe the action of pollutants moving past defenses.

Correct answer: penetrating

Explanation: The gerund 'penetrating' fits grammatically as it describes the action of pollutants moving deeply into the respiratory and circulatory systems. It logically follows the action of slipping past defenses.

Meaning: 'Penetrating' means to enter or pass through something, especially in a way that is difficult to stop.

Wrong options:

• contact: The noun 'contact' does not fit as it does not describe an action related to the movement of pollutants.
• slip: The verb 'slip' is already used in the previous blank, making it redundant here and does not describe the action of entering deeply.
• flashing: The gerund 'flashing' does not relate to the context of pollutants entering systems and implies a sudden display rather than a gradual process.
• breakthrough: The noun 'breakthrough' does not describe an action and is unrelated to the context of pollutants entering the body.

Correct answer: contributor

Explanation: The noun 'contributor' fits grammatically as it identifies fossil fuel combustion as a significant factor in air pollution. It aligns with the context of discussing causes of pollution.

Meaning: 'Contributor' refers to something that adds to or causes a particular result, in this case, air pollution.

Wrong options:

• contact: The noun 'contact' does not fit as it does not describe a cause or factor related to pollution.
• slip: The verb 'slip' does not fit grammatically as it does not describe a noun or factor contributing to pollution.
• flashing: The gerund 'flashing' does not relate to the context of pollution and does not describe a contributing factor.
• penetrating: The verb 'penetrating' does not fit as it describes an action rather than a noun that contributes to pollution.
• breakthrough: The noun 'breakthrough' refers to a significant advance, which does not describe a contributor to pollution.