Sentence Correction

16) Until quite recently, American economists have assumed that the unemployment rate being four per cent, there is a rough balance among jobs and job seekers.
(A) the unemployment rate being four per cent, there is a rough balance among jobs
and job seekers
(B) should the unemployment rate be four per cent, there is a rough balance among
jobs and job seekers
(C) were the unemployment rate four per cent, there is a rough balance between jobs
and job seekers
(D) if the unemployment rate is four per cent, there is a rough balance between jobs
and job seekers
(E) there is a rough balance among jobs and job seekers when there is an
unemployment rate that is four per cent

OA-D

I am convinced by OA. But not convinced by the explanation.

The reason to kick out C is " S-V Agreement problem(highlighted with bold)"

My Question is:

Isn't this subjunctive Mood, so in that case we must use were

Also, if you remember the famous phrase :

If I were a rich man

Here we are talking about a desire : something which has not happened in past, so technically aren't we creating a tense problem here(WERE=>Past Tense)

Thanks