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.