Why is an assembly line is a more efficient way of manufacturing?

In a perfectly competitive market, the long-run market supply curve tends to be horizontal or nearly so. what is another way to state this fact? (a) market supply is much more elastic in the long run than the short run. (b) in the long run, average total cost is minimized. (c) in the long run, price equals marginal cost. (d) market supply is much less elastic in the long run than the short run.