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?

Another one related to this.

Rather than continue to produce most of the items necessary for subsistence, a growing number of farm families during the first decades of the nineteenth century began to specialize in the production of grain or cotton and to use the cash proceeds from the sale of their crops to buy necessities

I've only posted the correct answer. So here "the cash proceeds from the sale"
Why not "sales"? One of the other answer choices was "the sales of their crops for buying"
This answer choice might be wrong for other reasons but I think GMAT wants you to distinguish between singular or plural

Stacey, Sorry read your rules after I posted and now it doesn't let u change the subject

Another sentence taken from correct answer.

It may be another fifteen years before spacecraft from Earth again venture to Mars,

Why not ventures?

Can someone please answer this.. will appreciate

Is the OA really C ? I know E is not perfect but I thought it might be the best choice.
Isn't there a case of an improper antecedent for the pronoun 'his' in option C ? The pronoun 'his' seems to refer to 'Diderot’s factual Encyclopedia'.

yes

possessive pronoun can refer to possessive noun. In this question, "his" is referring to Diderot’s

"his" can't refer to "Diderot’s factual Encyclopedia". possessive pronoun for encyclopedia would be "its"

"spacecraft" is a plural: one spacecraft, two spacecraft.
same goes for "aircraft".

you can tell that it's not singular because there's no article: i.e., it would have to be "a spacecraft" if it were singular.

i would say that "sale" is more idiomatic than "sales" IN THIS CONTEXT (please don't ever generalize anything about idiomatic usage!).
but there's actually another problem:
the correct answer says "use the cash proceeds ... to buy necessities", which is properly idiomatic;
the incorrect answer says "use the cash proceeds ... for buying necessities", which is unidiomatic.

correct.

RON but the PLURAL FORM of SPACECRAFT is SPACECRAFTS ???

similarly for AIRCRAFT AIRCRAFTS ??

Am i missing something big here ??

nope. see what i said above.

english has lots of annoying plurals like this - just think of them as more idioms that you have to memorize.