Read the dinner guest's dramatic recounting of a culinary disaster and select the grammatically correct sentence adverb.

_____, the smoke detector went off and forced us to evacuate the building before anyone had to actually taste my uncle's glowing green casserole.

The correct answer is Mercifully.

"Mercifully" functions here as a sentence adverb (specifically, an evaluative disjunct). Instead of describing how the smoke detector went off, it expresses the speaker's profound relief about the entire situation—it was a mercy that the alarm sounded and saved them from the casserole!

The other options do not express the speaker's attitude toward the event and do not make logical sense in this context.