68. 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

In choice A, the construction from hydrocarbons and nitrogen oxides ... react is ungrammatical. In B, the best choice, the conjunction when replaces the preposition/row, producing a grammatical and logical statement. In choice C, the use of the conjunction and results in the illogical assertion that the formation of ozone in the atmosphere happens in addition to, rather than as a result of, its formation when hydrocarbons and nitrogen oxide react with sunlight. Choice D omits the main verb, is, leaving a sentence fragment. E compounds the error of D with that of A.

c有什么问题，看不太明白解释？请nn们解释一下。a的from为什么说是不符合语法的？