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.
was at one time backed by
had at one time been backed by
was at one time backing
had at one time backed
has at one time been backed by

(A)

I'm still confused about this.

So can someone very clearly tell me exactly what the past perfect rule is. I thought the rule is as long as it references something in the past. Here - at one time - references something in the past.

In fact, I see many sentences that say, "At one time ... Before ... many days ago ..." So - these are all past actions. But - on this specific one - it's a matter that this sentence doesn't reference the present or another past time.

help?

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