In the context of Porter's Five Forces, which of the following scenarios best illustrates a high 'Threat of Substitutes' for a commercial airline operating short-haul domestic flights?

A rival airline launches a massive price-cutting campaign on the same routes.

The government completes a new high-speed rail network connecting major domestic cities.

A new startup airline enters the market with heavily discounted fares.

The sole supplier of aviation fuel significantly increases its prices.