Sentence Correction (SC) | Verb Tense - Past perfect question

The following question has B marked as the answer:

Although early novels first appeared as serials in newspapers and magazines, by the late eighteenth century they had been published as stand-alone books.

A. by the late eighteenth century they had been published
B. by the late eighteenth century they were published
C. by the late eighteenth century they were being published
D. by the time of the late eighteenth century they were published
E. by the late eighteenth century, they were publishing
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