Sentence Correction

Though the law will require emissions testing of all diesel vehicles, from tractor trailers to excursion buses, it will have no effect on sport utility vehicles, almost all of which are gasoline powered, and will not be subjected to emissions-control standards as stringent as they are for
diesel-powered vehicles.
A. powered, and will not be subjected to emissions-control standards as stringent as they are for
B. powered, and therefore not subjected to emissions-control standards that are as stringent as those of
C. powered and therefore not subject to emissions-control standards as stringent as those for
D. powered, which are not subject to emissions-control standards as stringent as they are for
E. powered and therefore they are not subject to emissions-control standards as stringent as those of

[spoiler]OA: C[/spoiler]
[spoiler]IMO: E[/spoiler]

I was able to cross out answer choice A and D, but found it difficult to get down to correct answer C.
Reason to cross out answer B: The comma + and indicates parallelism that result into deducing that
law will have no effect ......, and therefore not subjected (means law will not be subjected) -- This is incorrect as author wants to say that SUV(sport utility vehicles) will not be subjected ...

From answer choice c and e: I thought that it will be better if we put/use a pronoun as show in answer choice E 'they' -- (SUV)the meaning will be properly conveyed.

But I am confused whether we can use pronoun after therefore for such kind of example where the two clauses are connected by word and.

Please clarify the use of pronoun after therefore in such example and help me out to clarify my doubt regarding why answer choice E is wrong.

Source : GMAT Test paper set 25