as we know / as we know it / as is known to us

[outofdejavu]

Hi,

A test question goes:

The telephone ______ was the invention of Alexander Graham Bell.
(A) as is known to us
(B) as do we know it
(C) as we know it
(D) as we know

The given answer is said to be (C).
If (C) is indisputable, what are the flawed points of (A) and (D)?
If (C) is the correct wording in written text, would (D) be permissible in speech?

Please shed some light.
I would like to have native English speakers' comments.

[konungursvia]

C is wrong, as is B, because the pronoun "it" is not used in this way in English, though it is in the Latin languages. D is the most correct, A is not so much incorrect as odd.