Sentence Correction

Not only is the tensile strength of bamboo higher than that of mild steel, it withstands compression twice as well as concrete does. 

A) is the tensile strength of bamboo higher than that of mild steel, it

B) is bamboo's tensile strength higher than mild steel's, it

C) does bamboo have higher tensile strength than mild steel, it also

D) bamboo has higher tensile strength than mild steel, it also

E) does bamboo have higher tensile strength than that of mild steel, but it


source: e-gmat

Can any expert please tell me why option C is correct and D wrong?

I think option C is conspicuously wrong because "Does bamboo have higher tensile strength than mild steel" is more like a question.
D, in my opinion, is error-free and clear.

Also, can "Does bamboo have higher tensile strength than mild steel" be called a substantive clause? As per this article, the substantive clause typically begins with a relative pronoun or relative adverb.
Will it be wrong to construct a sentence like this:
Does bamboo have higher tensile strength than mild steel is not yet known.