The city has proposed a number of water treatment and

I was breaking my head over this question. It is not as easy as it sounds.

I believe I have a convincing answer.

A. the cost of which raises water bills high enough so that

enough so that is redundant

B. at a cost raising water bills so high that

- looks OK, but the participle phrase raising water bills could modify the noun cost or the subject projects. It is not very clear here. However the restrictive clause using that would avoid the ambiguity.

C. at a cost which raises water bills high enough so

This is idiomatic error. Usage is so + that

D. whose cost will raise water bills so high that

Whose clearly refers to the projects here. It also has the corect idiomatic usage

E. whose cost will raise water bills high enough so that

This has the redundancy problem.

If a thread is posted within a week I think it'll benefit everybody is all discussions can be grouped into the same thread rather than spreading out into several threads.