When designing IT systems for high availability, what is the difference between "vertical scaling" and "horizontal scaling"?

A Vertical scaling adds more machines to a system; horizontal scaling upgrades a single machine's resources B They are interchangeable terms for the same scaling strategy C Vertical scaling increases the capacity of a single machine (e.g., adding CPU/RAM); horizontal scaling adds more machines that work together to share the load D Vertical scaling only applies to storage devices, while horizontal scaling only applies to monitors