问题补充：
Usually a meteorological observatory (气象台) air quality in urban areas from one to five: excellent, fairly good, slightly polluted, poor, and dangerous.
A. identifies
B. purifies
C. classifies
D. clarifies
答案：
A. identifies
B. purifies
(答案→)C. classifies
D. clarifies
解析：考查动词：A. identifies识别，B. purifies使…纯洁，使…净化，C. classifies分类，分别，D. clarifies阐述，说明，句意：通常一个气象观测站把城市中的空气质量分类从一到五:非常好,相当好,轻微污染,差,和危险的。选C。
