Help the museum curator write the information plaque for the new T-Rex exhibit by choosing the correct verb form.

Every night after the visitors leave, the fragile dinosaur bones ___ with tiny toothbrushes to keep them pristine.

The correct answer is are cleaned.

We use the present simple passive (are cleaned) because this is a regular routine (indicated by "Every night"), and the bones are receiving the action, not performing it. "Are being cleaned" would mean it is happening right at this exact moment, while "were cleaned" is in the past.