Im reviewing some of the questions and also I got this one correct, I did it only because it sounded correct. If I thought any harder, I know that I can get it wrong.

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

D. had at one time backed

OA is A.

the explanation for why it is not D is as follows:

This choice incorrectly uses the past perfect tense and the active voice in the construction "had backed." The past perfect form (indicated by the word "had") should only be used in a sentence that contains two past actions or events; the earlier past action takes the past perfect tense, while the later past action takes the simple past tense. In this sentence, there is only one past event; therefore, the use of the past perfect tense is unnecessary and incorrect. Moreover, the active construction "had backed" distorts the meaning of the sentence. The original sentence indicated that the currency was backed by the fixed assets rather than, as is suggested here ("paper currency had . . . backed"), that the fixed assets were backed by the currency.

I'm thoroughly confused by the bolded part. can someone give me an example?