Question with-i

3.2 Form, Structure, and Sense - Modifier placement and clarity
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With its towering, six-spired exterior of granitelike quartz monzonite, the Salt Lake Temple is one of the most instantly recognizable structures in the state of Utah. However, many people do not know that blank built over the course of forty years, with construction beginning in 1853 and ending in 1893.

Which choice completes the text so that it conforms to the conventions of Standard English?

A.

they were

B.

both were

C.

it was

D.

one was