Aconstant-cost industry is one in which
- the long-run supply curve is perfectly inelastic.
- the long-run supply curve is downward sloping.
- the long-run supply curve is upward sloping.

None of the given option is correct.

Explanation:

A constant cost industry will have a perfectly elastic long-run supply curve. This is because in such industry the cost of inputs does not increase with the increase in demand. As a result, the increase in input level will not cause a rise in the price of factors or inputs.  

When the price of inputs remains the same, the cost curve, in the long run, will be a horizontal line.