A giant helium(氦) balloon is telling residents of Paris how good or bad the air quality is

问题补充：

A giant helium(氦) balloon is telling residents of Paris how good or bad the air quality is. When it’s good, the balloon ----measuring 22 meters wide and 32 meters high---shines green. , it’s red.

A. Though terrible

B. Whenever good

C. If worse

D. When bad

答案：

A. Though terrible

B. Whenever good

C. If worse

(答案→)D. When bad

解析：考查状语从句的省略。根据句子，可知前后是对比相反情况，所以前面是When it’s good,，后面应是When it is bad,it is 可以省略，所以选D。