A perfectly competitive industry with constant costs initially operates in long-run equilibrium. When demand decreases:

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A perfectly competitive industry with constant costs initiallyoperates in long-run equilibrium. When demand decreases:
in the short run, prices and profits will fall, but in the longrun, price will rise back to its initial level, as willprofits.
prices and profits will be lower than before the demand decreasein both the long run and the short run.
in the short run, prices and profits will be higher, but in thelong run, price will fall back to its original level, and firmswill again earn zero economic profit.
prices and profits will be higher than before the demanddecrease in both the long run and the short run