Question

in 2019, the international definition of a kilogram was changed to refer to plancks constant, an unchanging value foundational to quantum physics. previously, a kilogram defined as the mass of one particular object: a cylinder of platinum - iridium alloy called the international prototype of the kilogram (ipk). which choice completes the text so that it conforms to the conventions of standard english? a had b were c had been d have been

Explanation:

The past - perfect passive tense "had been" is used to show an action that was completed before another past action. Here, the definition of the kilogram as the mass of a particular object (the IPK) was an action that was in place before the change in 2019. "Were" is simple past and doesn't convey the earlier - than - another - past - action meaning. "Have been" is present perfect and not appropriate for this past - related context. "Had" is not the correct form as the sentence requires a passive construction.

Answer:

C. had been