This question is hard.
That said, what it tests is fairly basic: the permissible ways to join two independent clauses. A comma without a conjunction is NOT one of those permissible ways.
This sentence is similar to option C and is not correct:
Cats were the most popular pet until the nineteenth century, before that time dogs were the most popular pet.Here is another example similar to option C, also incorrect:
Women wore pants in the twentieth century, before that time they wore skirts.Let's fix that last sentence.
Correct:
Women wore pants in the twentieth century; before that time, women wore skirts.
(Two ICs joined by a semicolon. No coordinating conjunction needed.)
Correct:
Women wore pants in the twentieth century. Before that time, women wore skirts.
(Two ICs separated by a period.)
Correct:
Women wore pants in the twentieth century, but before that time, women wore skirts.
(Two ICs joined by [COMMA + BUT], in which
but is a "coordinating conjunction." Google the latter term if you do not understand it.)
You cannot join two independent clauses with just a comma. (Doing so creates a "comma splice.")
You must add a coordinating conjunction such as
but.Here is an easier example of a comma splice.
WRONG:
Dan was late, we left without him.I took that example from a site that explains comma splices and how to fix them.
The page on that site is easy to understand. You can find it
here.Finally,
the nineteenth century is a noun.
"Before which" is
very formal, but the phrase does not create a comma splice in which two ICs are "stuck together" with just a comma as is the case in (C).
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