Complete the local tour guide's dramatic story about the town's history.

Believe it or not, this boring dental clinic ___ a terrifying haunted mansion back in the 1980s!

The correct answer is used to be.

"Used to" shows that a state existed in the past but doesn't anymore. Because we need a main verb to connect the subject (clinic) to the noun phrase (a terrifying haunted mansion), we must include the verb "be."