—What does PM 2.5 mean?—It's a professional indicating the condition of air pollution.A.

问题补充：

—What does PM 2.5 mean?

—Its a professional indicating the condition of air pollution.

A. term

B. item

C. number

D. mark

答案：

(答案→)A. term

B. item

C. number

D. mark

解析：选A考查名词辨析。句意：“PM 2.5是什么意思？”“它是一个表示空气污染状况的专业术语。”term“术语”，符合句意。item“项目”；number“数字”；mark“记号”。