GMAT Sentence Correction (SC) | Re: Stereos as already a standard feature on many new automobiles

Stereos as already a standard feature on many new automobiles, more car companies are including CD players and multi-disk changers as added incentives to buyers.

(A) Stereos as(Lack of main verb is missing in the first part of the sentence)

(B) As stereos(Same as A)

(C) As stereos being(Same as A)

(D) Since stereos are(Correct) (Main verb 'are' is present)

(E) Since stereos being(Same as A)

Kudos please if you like my explanation.
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