单选题 Smoke particles and other air pollutants are often trapped in the atmosphere, thus forming dirty fog.
A. constrained B. caught C. concealed D. concentrated

【正确答案】 B
【答案解析】[解析] 题干中的trap意为“困住”。A项“约束”；B项“抓住”；C项“隐藏”；D项“专心于”，只有B项与题干种划线单词意义相近。