Ozone

ceilidh.erickson wrote:Binit's explanation was perfectly on point.

As a more general rule, we can look at the differences between the answer choices to tell us what is being tested.

A, B, and C begin with "ozone is formed," while D and E have "ozone, formed..." If some answer choices express the idea of "formed" as a verb and some express it as a modifier, we then ask ourselves - should that be a verb? Or does the subject "ozone" have another verb elsewhere? As Binit pointed out, the only other verb is "react," which cannot go with the singular "ozone." We can rule out D and E.

We could then look to the difference between "from" and "when." "From" would be introducing a prepositional phrase, while "when" would be introducing a subordinate clause. Because we have the verb "react" in the non-underlined portion, it must be a clause, not a prepositional phrase. Eliminate A.

Now, the final difference between B and C is "and." When we have a conjunction (e.g. "and") after an independent clause (in this case "ozone is formed"), there must also be an independent clause after it. The "when" makes the second clause subordinate, though, so C is a fragment.

The answer must be B.