Sentence Correction

The Cold War, an apt name for the nervous post-World War II relations between the superpowers, prompted families to build bomb shelters costing thousands of dollars but fortunately were never used.

A) families to build bomb shelters costing thousands of dollars but

B) families to invest in bomb shelters that would cost thousands of dollars and that

C) families to invest thousands of dollars in bomb shelters that

D) the building by families of bomb shelters costing thousands of dollars that

E) the building of bomb shelters by families that cost thousands of dollars but

the reason i eliminated A was because nothing was parallel to "but fortunately were never used" .as "but" is a parallel marker we need a parallel structure across "but"
also in option B :we are making two things parallel when we actually don't need a parallelism between them : bomb shelters that would cost thousands of dollars and that fortunately were never used.

kindly tell me if my logic is correct
thanks and regards

The Cold War, an apt name for the nervous post-World War II relations between the superpowers, prompted families to build bomb shelters costing thousands of dollars but fortunately were never used.

A) families to build bomb shelters costing thousands of dollars but -
Parallelism issue
costing thousands of dollars (present participle phrase) & fortunately were never used (clause)

.

B) families to invest in bomb shelters that would cost thousands of dollars and that - Error with tense

C) families to invest thousands of dollars in bomb shelters that (correct)

D) the building by families of bomb shelters costing thousands of dollars that
too many issues for example that refers to dollar etc

E) the building of bomb shelters by families that cost thousands of dollars but -- families dont cost 1000s of dollars