GMAT Sentence Correction (SC) | Re: Although early novels first appeared as serials in newspapers and maga

Source: McGraw Hill GMAT

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 them