Published during the late eighteenth century, Diderot’s factual Encyclopedia and his friend Voltaire’s fictional Candide were the cause of such a sensational scandal, and both men prudently chose to embark on extended vacations in nearby Austria.
(A) Diderot’s factual Encyclopedia and his friend Voltaire’s fictional Candide were the cause of such a sensational scandal, and
(B) Diderot and his friend Voltaire’s caused such a sensational scandal with their factual Encyclopedia and fictional Candide, respectively, that
(C) Diderot’s factual Encyclopedia and his friend Voltaire’s fictional Candide were the cause of a scandal so sensational that
(D) the scandal caused by Diderot’s factual Encyclopedia and his friend Voltaire’s fictional Candide was so sensational
(E) a factual Encyclopedia by Diderot and the fictional Candide, by his friend Voltaire, caused a sensational scandal, which
OA for this is C. But confused as in were the cause in C should be "were the causes"?
So if structure is like, "A and B are the X of", shouldn't X be plural?