Often visible as smog, ozone is formed in the atmosphere from hydrocarbons and nitrogen oxides two major pollutants emitted by automobiles, react with sunlight.
(A)ozone is formed in the atmosphere from
(B)ozone is formed in the atmosphere when
(c)ozone is formed in the atmosphere, and when
(D)ozone , formed in the atmosphere when
(E)ozone , formed in the atmosphere from
题目分析：如果不加be动词，句子不完整，干掉D和E。C的那个and和前面在语义上没有对应的并列关系，干掉。由于后面react是个动词，则前面不能用from这个介词引导，否则无法形成一个句子，只有when后面引导了一个句子，保证react可以作为从句的谓语出现。选B
