How to read this
Each firm's marginal cost depends on the other firm's output: with pollution or congestion, more raises firm 1's marginal cost. For a fixed output of the other firm, adding the supply curves gives the dotted lines. But when the price rises both firms expand and push up each other's costs, so the true market supply (solid) is steeper.
- Raise : cuts across the dotted lines.
- Switch to Positive (training, networks, infrastructure): market supply becomes flatter.