Final answer:
In the presence of an external benefit and no external cost, the social cost curve is positioned below the supply curve, reflecting additional value to society not included in private production costs.
Step-by-step explanation:
In the case of an external benefit (but no external cost), the social cost curve lies below the supply curve.
This happens because the external benefit represents additional value to society that is not captured in the private costs of production.
Producers do not take these social benefits into account when deciding how much to produce. Consequently, the market equilibrium quantity will be lower than the socially optimal quantity.
If these external benefits could be internalized, say through government intervention such as subsidies, then the supply curve would reflect these social benefits and would shift downward, aligning closer to the social cost curve.