Explain why the "industry supply curve" is a horizontal line?

In the Classical economics, perfect employment is a premise in the labor market.

Because the number of workers employing in perfect employment is constant,the volume of production is maintained the same.

（Volume of production corresponding to the perfect employment）

In other words the total supply curve becomes perpendicular

because AS is constant at a perfect employment national income standard

The price level of the substance becomes the same price level in the long term.

The definition of a perfectly competitive market is one in which no firm has any market power. Market power means the ability to influence the market price of the good. If no firm has the ability to influence the price of the good, that means the demand curve facing each individual firm must be perfectly elastic (horizontal). Note that this doesn't mean that the market demand curve is horizontal; the market demand curve is still downward sloping. Consider this example: You're a bottled water manufacturer competing against 9 other bottled water manufacturers. You each sell identical water: same sizes, same bottles, no vitamin-enhanced water, no flavors, etc. In other words, your industry is perfectly competitive. If all manufacturers sell at $1.00 per bottle, and you raise your price at all, even to $1.01 per bottle, there's no reason anyone would ever buy from you. You would lose all of your business to another firm. Therefore, the demand curve facing you is perfectly elastic. However, if plastic becomes much cheaper (one of the inputs to the bottles in bottled water), everyone's supply curves will shift to the right, and the market price will decrease. You won't be able to charge anything different than the new market price, but the market price can still move.

in a chart everything goes horizontal?

normally the horizontal "x" axis is the time axis

so if nothing happens what so ever, the line is completely flat over time, horizontally

if everything happens in this amount of time, the line would still move horizontally but also go way higher on the y axis as well

either your question is dumb or im not getting the question

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