sentence construction - A multiple choice question

Extracted from a textbook of master.get.com.tw

17. ______ us the ability to resist disease, but also helps us build our body tissues.
    (A) Vitamin C not only provides
    (B) Not only does vitamin C provide
    (C) Not only vitamin C provides
    
    (D) Vitamin C, which not only provides

The answer of question number 17 is A, which I know is correct, but Why can't I choose D? They both seem Ok to me.

Thanks!

Answer

Because but also provides requires a verb in the preceding clause which it is countering. but joins two clauses both of which must be able to stand on their own. If the first lacks a verb, it is not a clause. With option D, there is no such verb. The verb there is in the which-clause not in the main clause. A verb in a relative clause does not act as a verb in the main clause.

Consider:

Bill, who lives in London, but also has lived in Paris.^{ungrammatical}

Bill lives in London but has also lived in Paris.

Here, with not only:

Bill, who lives not only in London, but also has a house in Paris.^{ungrammatical}.

Bill lives not only in London but also has a house in Paris.

The subject here, I, lacks a verb:

I, who eat fish, but also like meat. ^{ungrammatical}

Grammatical:

I eat fish but also like meat.

Or you can eliminate but:

I, who eat fish, also like meat.