GMAT Sentence Correction (SC) | Re: Though most paper currency was at one time backed by fixed assets such

Though most paper currencywas at one time backed by fixed assets such as gold or silver, it now derives its purchasing power from a declaratory fiat of the issuing government.

(A)was at one time backed by

(B) had at one timebeen backed by

(C) was at one timebacking

(D)had at one timebacked

(E) has at one timebeen backed by