Final answer:
The long-run supply curve under pure competition can be horizontal, upward sloping, or downward sloping depending on the industry's cost structure.
Step-by-step explanation:
The long-run supply curve under pure competition can take different shapes depending on the industry's cost structure. In a constant-cost industry, where costs remain the same regardless of output level, the long-run supply curve is a horizontal line.
This means that firms can increase their output without affecting the price. In an increasing cost industry, where costs rise as output increases, the long-run supply curve is upward sloping. Finally, in a decreasing cost industry, where costs decrease as output increases, the long-run supply curve is downward sloping.