Which market structure allows firms to go on earning supernormal profits in the long run, because significant barriers prevent new firms from entering to compete the profits away?

monopoly, where high barriers to entry block potential competitors.

In the short run, at what point does a perfectly competitive firm choose to shut down production rather than continue operating?

When price falls below average variable cost, so operating would fail to cover even variable costs.

Price equal to average total cost, so total revenue exactly covers all costs including a normal return to capital.

A Level Economics

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