Which of the following is a run-on sentence?

Adon't be put off by the historical aspect; this is a riveting book filled with romance.

Bsome readers might find the book a bit long it hasalmost a thousand pages.

Cthe novel is set during the middle ages and the author is very attentive to historical accuracy.

Dthe process of constructing a cathedral during this time was extremely complicated and involved the skills of many artisans with specific expertise?