Meaning: 'Sources' refers to the origins or causes of something, in this case, the origins of particle pollution.

Wrong options:

• somatic: The term 'somatic' relates to the body and does not fit the context of pollution sources.
• stymie: 'Stymie' means to obstruct or hinder, which does not apply to the context of identifying sources of pollution.
• sundry: 'Sundry' means various or diverse, but it does not serve as a noun that can act as a subject in this context.

Correct answer: primary

Explanation: The word 'primary' is an adjective that describes the type of sources being referred to in the context of particle pollution. It fits grammatically as it modifies the noun 'sources' and is consistent with the earlier mention of primary sources.

Meaning: 'Primary' means first or main, indicating that these sources are the principal contributors to particle pollution.

Wrong options:

• patience: 'Patience' refers to the ability to wait without frustration, which is irrelevant in the context of pollution sources.
• performance: 'Performance' relates to how well something functions, which does not apply to the context of sources of pollution.
• paraphrase: 'Paraphrase' means to restate something in different words, which does not fit the context of identifying types of sources.

Correct answer: harmful

Explanation: The word 'harmful' is an adjective that describes the negative impact of particle pollution on health. It fits grammatically as it complements the verb 'can be' and aligns with the context of discussing health effects.

Meaning: 'Harmful' means causing or capable of causing damage or injury, indicating that particle pollution poses risks to health.

Wrong options:

• habitat: 'Habitat' refers to the natural environment of an organism, which does not relate to the effects of pollution on health.
• harvest: 'Harvest' refers to the process of gathering crops, which is unrelated to the context of health effects from pollution.
• hence: 'Hence' is an adverb meaning 'for this reason,' which does not fit grammatically or contextually in this sentence.

Correct answer: sites

Explanation: The word 'sites' is a plural noun that refers to locations where activities occur, fitting grammatically as it aligns with the context of discussing places contributing to PM10 pollution. It matches the structure of the sentence and maintains coherence with the preceding examples.

Meaning: 'Sites' refers to specific locations or areas, in this case, where dust and pollution are generated.

Wrong options:

• scrutinize: 'Scrutinize' means to examine closely, which does not fit as a noun in this context.
• safety: 'Safety' refers to the condition of being protected from harm, which does not apply to the context of locations contributing to pollution.
• sarcastic: 'Sarcastic' describes a form of verbal irony, which is irrelevant in the context of discussing pollution sites.