The unfinished sentence

An original narrative passage about an invented examination-hall incident.

At the front of the hall, the clock measured out the last twenty minutes. Beneath it, the invigilator sat with her hands folded beside a stack of spare paper. Rows of candidates bent over their desks. Pens moved, pages turned and, at regular intervals, someone coughed into a sleeve.

The alarm began with a single hard cry. Every pen stopped. Before anyone could decide whether the sound belonged inside the hall or beyond it, the cry came again. It was exactly the same: loud, level and without hurry. The clock continued to move, but nobody looked at it now.

The invigilator stood and raised one hand. “Please remain—” she began, but the alarm cut the instruction into pieces. Its metal voice filled the pause before she tried again. Candidates looked towards her, then towards the doors, waiting to discover which command they were expected to obey.

On the third attempt, she pointed to the nearest exit. The straight rows immediately came apart. Chairs knocked against desk legs, papers lifted in the air made by passing bodies, and the narrow central aisle widened as everyone pressed towards it. One candidate tried to place a ruler over an unfinished page. The ruler slid aside, and the page turned itself face down.

In the corridor, the alarm was stronger. It bounced from the low ceiling and followed the line down the stairs. Nobody spoke above it. Even outside, in the open yard, its repeated call pushed through the walls of the building. The candidates stood in loose groups, but their attention remained fixed on the windows, behind which the empty hall flashed at regular intervals.

Then the alarm stopped. Its absence seemed larger than ordinary silence. A few people began to talk, yet their voices were careful, as if the alarm had merely paused to listen. When the candidates returned, the clock had been covered and fresh paper waited on the front desk. They sat in their former places, but the rows took time to become still.

The unfinished page remained face down until the final instruction was given. When it was turned over, the last sentence ended halfway across the line. The pen hovered above the missing words, unable to recover the thought that the alarm had carried away.

How does the writer present the alarm's challenge to the invigilator's authority and the changes in the candidates' behaviour? Support your answer with references to the passage.