Though most paper currency was 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 time been backed by

(C) was at one time backing

(D) had at one time backed

(E) has at one time been backed by

Could some experts help me find the wrong in option B?

OA A